Cassini Catches Titan Naked in the Solar Wind

This diagram depicts conditions observed by NASA's Cassini spacecraft during a flyby in Dec. 2013, when Saturn's magnetosphere was highly compressed, exposing Titan to the full force of the solar wind. Image credit: NASA/JPL-Caltech

Researchers studying data from NASA's Cassini mission have observed that Saturn's largest moon, Titan, behaves much like Venus, Mars or a comet when exposed to the raw power of the solar wind. The observations suggest that unmagnetized bodies like Titan might interact with the solar wind in the same basic ways, regardless of their nature or distance from the sun.

Titan is large enough that it could be considered a planet if it orbited the sun on its own, and a flyby of the giant moon in Dec. 2013 simulated that scenario, from Cassini's vantage point. The encounter was unique within Cassini's mission, as it was the only time the spacecraft has observed Titan in a pristine state, outside the region of space dominated by Saturn's magnetic field, called its magnetosphere.

"We observed that Titan interacts with the solar wind very much like Mars, if you moved it to the distance of Saturn," said Cesar Bertucci of the Institute of Astronomy and Space Physics in Buenos Aires, who led the research with colleagues from the Cassini mission. "We thought Titan in this state would look different. We certainly were surprised," he said.

The solar wind is a fast-flowing gale of charged particles that continually streams outward from the sun, flowing around the planets like islands in a river. Studying the effects of the solar wind at other planets helps scientists understand how the sun's activity affects their atmospheres. These effects can include modification of an atmosphere's chemistry as well as its gradual loss to space.

Titan spends about 95 percent of the time within Saturn's magnetosphere. But during a Cassini flyby on Dec. 1, 2013, the giant moon happened to be on the sunward side of Saturn when a powerful outburst of solar activity reached the planet. The strong surge in the solar wind so compressed the sun-facing side of Saturn's magnetosphere that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to, and unprotected from, the raging stream of energetic solar particles.

Using its magnetometer instrument, which is akin to an equisitely sensitive compass, Cassini has observed Titan many times during the mission's decade in the Saturn system, but always within Saturn's magnetosphere. The spacecraft has not been able to detect a magnetic field coming from Titan itself. In its usual state, Titan is cloaked in Saturn's magnetic field.

This time the influence of Saturn was not present, allowing Cassini's magnetometer to observe Titan as it interacted directly with the solar wind. The special circumstance allowed Bertucci and colleagues to study the shockwave that formed around Titan where the full-force solar wind rammed into the moon's atmosphere.

At Earth, our planet's powerful magnetic field acts as a shield against the solar wind, helping to protect our atmosphere from being stripped away. In the case of Venus, Mars and comets -- none of which is protected by a global magnetic field -- the solar wind drapes around the objects themselves, interacting directly with their atmospheres (or in the comet's case, its coma). Cassini saw the same thing at Titan.

Researchers thought they would have to treat Titan's response to the solar wind with a unique approach because the chemistry of the hazy moon's dense atmosphere is highly complex. But Cassini's observations of a naked Titan hinted at a more elegant solution. "This could mean we can use the same tools to study how vastly different worlds, in different parts of the solar system, interact with the wind from the sun," Bertucci said.

Bertucci noted that the list of similarly unmagnetized bodies might include the dwarf planet Pluto, to be visited this year for the first time by NASA's New Horizons spacecraft.

"After nearly a decade in orbit, the Cassini mission has revealed once again that the Saturn system is full of surprises," said Michele Dougherty, principal investigator of the Cassini magnetometer at Imperial College, London. "After more than a hundred flybys, we have finally encountered Titan out in the solar wind, which will allow us to better understand how such moons maintain or lose their atmospheres."

The new research is published today in the journal Geophysical Review Letters.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The magnetometer team is based at Imperial College, London, U.K.

Source: Nasa

The Ancient exoplanet discovery boosts chances of finding alien life

An artist's impression of the oldest known system of terrestrial-sized planets, Kepler-444. Tiago Campante/Peter Devine, University of Birmingham, Author provided Credit: By Daniel Huber, Astronomer at University of Sydney

One of the crucial variables in calculating the likelihood that alien life exists elsewhere in our galaxy is the number of stars that possess planetary systems, and the proportion of those planets that might be suitable for life.

So the discovery of no less than five sub-Earth-sized exoplanets orbiting an ancient star, Kepler-444, which is not too distant from our own solar system, has significant ramifications for the possibility we might one day run into ET.

Formed over 11-billion years ago, the Kepler-444 system proves that such small planets have existed through most of the history of our universe. And the more small planets that exist, the higher the chances that one of them (or one of their moons) might sit in the so-called “Goldilocks zone” that enables life to exist.

This remarkable discovery was made possible not only by the space-based NASA Kepler telescope but also a technique called asteroseismology.

Kepler continuously measured the brightness of more than 150,000 stars for four years. As planets orbit in front of the stellar disc they cause small dips in the brightness of the star, yielding information on the planet’s orbital period and size relative to the size of their host star.

More than 1,800 exoplanets have been discovered to date, including some Earth-sized planets in the habitable zone. Such discoveries have demonstrated that planets with favourable conditions for life may actually be common.

But the age of the host stars – and therefore the age of the planets – was often unknown. This is because the clues that give a hint to the age of a star tend to be hidden beneath its visible surface.

Using asteroseismology to date a star

An artist’s impression of Kepler-10, illustrating the paths of sound waves in the stellar interior which can be used to determine the fundamental properties – including age – of planet host stars. Gabriel Perez Diaz, Instituto de Astrofisica de Canarias

Fortunately, the variability in the brightness of stars offers a way to resolve this problem using asteroseismology.

Stars with similar and cooler temperatures than our sun transport energy to their surface through the up-flow and down-flow of gas that flows due to the interplay of buoyancy and gravity. The turbulent motion of the gas excites pressure waves to travel through the stellar interior.

The frequency of these waves – also referred to as oscillations – are determined by the sound speed, which in turn depends on the stellar interior structure and composition.

These oscillations also travel to different depths within the star, thereby offering a way to probe the structure by observing the oscillations. As the core properties of the star change with time, such changes are imprinted in the oscillation frequency patterns.

Conveniently, we can measure stellar oscillations using the same data we use to discover transiting planets. Thus we were able to use asteroseismology to study a fascinating planetary system in exquisite detail and to determine the age of the host star.


Kepler-444: An ancient laboratory for planetary and stellar astrophysics

Comparison of the sizes of inner solar system planets to the planets discovered in the Kepler-444 system. Daniel Huber & NASA

Unlike our solar system, however, the Kepler-444 planets orbit their host star in less than 10 days. Even taking into account the cooler temperature of Kepler-444 compared to our sun, this places these ancient planets well outside the habitable zone.

Despite the rather hostile environment, Kepler-444 marks an important milestone to understand whether life may be common outside the solar system. While the Kepler mission has previously demonstrated that small planets are abundant, Kepler-444 proves that such planets have formed for most of the history of our universe.

If life can form on Earth-sized planets in the habitable zone of other stars, this implies that it may have formed on distant planets long before life emerged here on Earth.

Source: University of Sydney

Astronomers discover a replica solar system

This image shows Kepler-444 and its five orbiting planets. (Courtesy of Peter Devine and Tiago Campante/University of Birmingham)

Scientists have located an ancient solar system, dating back to the dawn of the galaxy, which appears to be a miniature version of the inner planets in our own solar system.

An international research group, including Yale University professors of astronomy Sarbani Basu and Debra Fischer, announced the discovery Jan. 27 in The Astrophysical Journal. The findings are the result of observations made by the NASA Kepler spacecraft over a period of four years.

The old, Sun-like star, named Kepler-444, has five orbiting planets with sizes between those of Mercury and Venus. Kepler-444 formed 11.2 billion years ago, when the universe was less than 20% of its current age. This makes Kepler-444 the oldest known system of terrestrial-sized planets. The Kepler-444 system was already older than our own solar system is today when our Sun and planets were born.

“This system shows that planet formation could take place under very different conditions from the ones in which our solar system was formed and has implications for estimating the total number of planets in our galaxy, and other galaxies,” Basu said.

The five planets in the Kepler-444 system have orbits that are equivalent to less than one-tenth of Earth’s distance from the Sun. The Kepler-444 planets are rocky and Earth-like, but their exact compositions are uncertain.

The scientists carried out their research using asteroseismology — listening to the host star’s natural resonances, which are caused by sound trapped within it. These oscillations lead to miniscule changes or pulses in the star’s brightness, allowing researchers to measure the star’s diameter, mass, and age. The planets were then detected from the dimming that occurs when the planets transited, or passed across, the stellar disc. This fractional fading in the intensity of starlight enabled scientists to measure accurately the sizes of the planets relative to the size of the star.

“There are far-reaching implications for this discovery,” said lead author Tiago Campante of the University of Birmingham (U.K.). “We now know that Earth-sized planets have formed throughout most of the universe’s 13.8-billion-year history, which could provide scope for the existence of ancient life in the galaxy.”

The research collaboration involved nearly two-dozen institutions in the United States, England, Denmark, Portugal, Australia, Germany, and Italy.

Source: Yale university

Tail discovered on long-known asteroids

The faint tail can be seen in active asteroid 62412. Image courtesy of Scott Sheppard

Washington, D.C.--A two-person team of Carnegie's Scott Sheppard and Chadwick Trujillo of the Gemini Observatory has discovered a new active asteroid, called 62412, in the Solar System's main asteroid belt between Mars and Jupiter. It is the first comet-like object seen in the Hygiea family of asteroids. Sheppard will present his team's findings at the American Astronomical Society's Division of Planetary Sciences meeting and participate today in a press conference organized by the society.

Active asteroids are a newly recognized phenomenon. 62412 is only the 13th known active asteroid in the main asteroid belt. Sheppard and Trujillo estimate that there are likely about 100 of them in the main asteroid belt, based on their discovery.

Active asteroids have stable orbits between Mars and Jupiter like other asteroids. However, unlike other asteroids, they sometimes have the appearance of comets, when dust or gas is ejected from their surfaces to create a sporadic tail effect. Sheppard and Trujillo discovered an unexpected tail on 62412, an object which had been known as a typical asteroid for over a decade. Their findings reclassify it as an active asteroid. The reasons for this loss of material and subsequent tail in active asteroids are unknown, although there are several theories such as recent impacts or sublimation from solid to gas of exposed ices.

"Until about ten years ago, it was pretty obvious what a comet was and what a comet wasn't, but that is all changing as we realize that not all of these objects show activity all of the time," Sheppard said.

In the past, asteroids were thought to be mostly unchanging objects, but an improved ability to observe them has allowed scientists to discover tails and comas, which are the thin envelope of an atmosphere that surrounds a comet's nucleus.

"We're actually looking anew through our deep survey at a population of objects that other people cannot easily observe, because we're going much deeper," Sheppard said, explaining why they were able to see that 62412 was active when it had been considered a typical main belt asteroid for 15 years.;

Discoveries such as this one can help researchers determine the processes that cause some asteroids to become active. Sheppard will discuss his and Trujillo's theories about the genesis of 62412's activity. They found that 62412 has a very fast rotation that likely shifts material around its surface, some of which may be emitted to form the comet-like appearance. The tail may be created directly from ejected material off the fast rotating nucleus, or from ice within the asteroid subliming into water vapor after being freshly exposed on the surface. They also find a density for 62412 typical of primitive asteroids and not consistent with the much lower-density comets. Further monitoring of this unusual object will help confirm the activity's source.

Sheppard and Trujillo have a paper about this work in press at The Astronomical Journal.


Source: NASA

New instrument reveals recipe for other Earths

How do you make an Earth-like planet? The 'test kitchen' of Earth has given us a detailed recipe, but it wasn't clear whether other planetary systems would follow the same formula. Now, astronomers have found evidence that the recipe for Earth also applies to terrestrial exoplanets orbiting distant stars. Credit: David A. Aguilar (CfA)

How do you make an Earth-like planet? The "test kitchen" of Earth has given us a detailed recipe, but it wasn't clear whether other planetary systems would follow the same formula. Now, astronomers have found evidence that the recipe for Earth also applies to terrestrial exoplanets orbiting distant stars.

"Our solar system is not as unique as we might have thought," says lead author Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA). "It looks like rocky exoplanets use the same basic ingredients."

Dressing presented the research today at a meeting of the American Astronomical Society.

The key to the discovery was the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands. (HARPS stands for High-Accuracy Radial velocity Planet Searcher.) It is designed to accurately measure the masses of small, Earth-sized worlds. Those measurements are crucial to determine densities and therefore compositions.

"Our strategy for using HARPS-North over the past year has been to focus on planets less than two times the diameter of Earth and to study a few planets really well," explains Harvard astronomer David Charbonneau (CfA), who currently heads up the HARPS-North Science Team.

Most recently the team targeted Kepler-93b, a planet 1.5 times the size of Earth in a tight, 4.7-day orbit around its star. The mass and composition of this world were uncertain. HARPS-North nailed the mass at 4.02 times Earth, meaning that the planet has a rocky composition.

The researchers then compared all ten known exoplanets with a diameter less than 2.7 times Earth's that had accurately measured masses. They found that the five planets with diameters smaller than 1.6 times Earth showed a tight relationship between mass and size. Moreover, Venus and Earth fit onto the same line, suggesting that all these worlds have similar rock-iron compositions.

As for the larger and more massive exoplanets, their densities proved to be significantly lower, meaning that they include a large fraction of water or other volatiles, hydrogen and/or helium. They also showed more diverse compositions rather than fitting into a single group like the smaller terrestrial worlds.

The team also noted that not all planets less than six times the mass of Earth are rocky. Some low-mass worlds with very low densities are known (such as the planets in the Kepler-11 system). But for typical close-in small planets, the chances are high that they share an Earth-like composition.

"To find a truly Earth-like world, we should focus on planets less than 1.6 times the size of 

Earth, because those are the rocky worlds," recommends Dressing.

Making Other Earths

Makes one small model planet

Ingredients:

• 1 cup magnesium
• 1 cup silicon
• 2 cups iron
• 2 cups oxygen
• 1/2 teaspoon aluminum
• 1/2 teaspoon nickel
• 1/2 teaspoon calcium
• 1/4 teaspoon sulfur
• dash of water delivered by asteroids

Blend well in a large bowl, shape into a round ball with your hands and place it neatly in a habitable zone area around a young star. Do not over mix. Heat until mixture becomes a white hot glowing ball. Bake for a few million years. Cool until color changes from white to yellow to red and a golden-brown crust forms. It should not give off light anymore. Season with a dash of water and organic compounds. It will shrink a bit as steam escapes and clouds and oceans form. Stand back and wait a few more million years to see what happens. If you are lucky, a thin frosting of life may appear on the surface of your new world.

Source: Harvard-Smithsonian Center for Astrophysics

Super-Earths have long-lasting oceans

This artist's depiction shows a gas giant planet rising over the horizon of an alien waterworld. New research shows that oceans on super-Earths, once established, can last for billions of years.
Credit: David A. Aguilar (CfA)

For life as we know it to develop on other planets, those planets would need liquid water, or oceans. Geologic evidence suggests that Earth's oceans have existed for nearly the entire history of our world. But would that be true of other planets, particularly super-Earths? New research suggests the answer is yes and that oceans on super-Earths, once established, can last for billions of years.

"When people consider whether a planet is in the habitable zone, they think about its distance from the star and its temperature. However, they should also think about oceans, and look at super-Earths to find a good sailing or surfing destination," says lead author Laura Schaefer of the Harvard-Smithsonian Center for Astrophysics (CfA).

Schaefer presented her findings today in a press conference at a meeting of the American Astronomical Society.

Even though water covers 70 percent of Earth's surface, it makes up a very small fraction of the planet's overall bulk. Earth is mostly rock and iron; only about a tenth of a percent is water.

"Earth's oceans are a very thin film, like fog on a bathroom mirror," explains study co-author Dimitar Sasselov (CfA).

However, Earth's water isn't just on the surface. Studies have shown that Earth's mantle holds several oceans' worth of water that was dragged underground by plate tectonics and subduction of the ocean seafloor. Earth's oceans would disappear due to this process, if it weren't for water returning to the surface via volcanism (mainly at mid-ocean ridges). Earth maintains its oceans through this planet-wide recycling.

Schaefer used computer simulations to see if this recycling process would take place on super-Earths, which are planets up to five times the mass, or 1.5 times the size, of Earth. She also examined the question of how long it would take oceans to form after the planet cooled enough for its crust to solidify.

She found that planets two to four times the mass of Earth are even better at establishing and maintaining oceans than our Earth. The oceans of super-Earths would persist for at least 10 billion years (unless boiled away by an evolving red giant star).

Interestingly, the largest planet that was studied, five times the mass of Earth, took a while to get going. Its oceans didn't develop for about a billion years, due to a thicker crust and lithosphere that delayed the start of volcanic outgassing.

"This suggests that if you want to look for life, you should look at older super-Earths," Schaefer says.

Sasselov agrees. "It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet's atmosphere. So, it takes time for life to become detectable."

This also suggests that, assuming evolution takes place at a similar rate to Earth's, you want to search for complex life on planets that are about five and a half billion years old, a billion years older than Earth.

Source: Harvard-Smithsonian Center for Astrophysics

Unseen planet revealed by its gravity

Using Kepler Telescope transit data of planet “b”, scientists predicted that a second planet “c” about the mass of Saturn orbits the distant star KOI-872. This research, led by Southwest Research Institute and the Harvard-Smithsonian Center for Astrophysics, is providing evidence of an orderly arrangement of planets orbiting KOI-872, not unlike our own solar system. Credit: Southwest Research Institute

More than a 150 years ago, before Neptune was ever sighted in the night sky, French mathematician Urbain Le Verrier predicted the planet's existence based on small deviations in the motion of Uranus. In a paper published May 10 in the journal Science online, a group of researchers led by Dr. David Nesvorny of Southwest Research Institute has inferred another unseen planet, this time orbiting a distant star, marking the first success of this technique outside the solar system.

Using a laborious computational method to assess the effects of gravity, known as gravitational perturbation theory, Le Verrier argued in favor of Neptune's existence and predicted the position of this hidden world to within an arc degree, as later detected directly by Johann Galle of the Berlin Observatory.

"Today's telescopes are detecting planets around distant stars, and NASA's Kepler Telescope, launched in 2009, is a champion among them," says Nesvorny. It finds planets by continuously monitoring the brightness of more than 150,000 stars, searching for brief periods of time, known as transits, when a star appears fainter because it is obscured by a planet passing in the foreground. But there's a twist.

"For a planet following a strictly Keplerian orbit around its host star, the spacing, timing and other properties of the observed transit light curve should be unchanging in time," said Dr. David Kipping of the Harvard-Smithsonian Center for Astrophysics and second author of the paper. "Several effects, however, can produce deviations from the Keplerian case so that the spacing of the transits is not strictly periodic."

A hidden planet, for example, can distort the sequence of transits if it gravitationally pulls on the transiting planet and delays some transits relative to others.

As part of the Hunt for the Exomoons with Kepler (HEK) project, the team analyzed recently released Kepler data and identified systems with transiting planets that show transit variations indicative of hidden companions, such as unseen moons or planets. The team identified the Sun-like star known as KOI-872 (KOI stands for Kepler Objects of Interest) as exceptional in that it shows transits with remarkable time variations over two hours.

"It quickly became apparent to us that a large hidden object must be pulling on the transiting planet," says Nesvorny. "To put this in context, if a bullet train arrives in a station two hours late, there must be a very good reason for that. The trick was to find what it is."

Using Le Verrier's perturbation theory to speed up time-consuming computer calculations of many possible configurations of planetary orbits, the HEK team showed that the observed variations can be best explained by an unseen planet about the mass of Saturn that orbits the host star every 57 days. According to the analysis, the planetary orbits are very nearly coplanar and circular, reminiscent of the orderly arrangement of orbits in our solar system.

The team's claim will be put to the test by Kepler's new observations, which will track dozens of new transits of KOI-872, comparing their timing to published predictions.

"Whilst the principal goal of the HEK project will continue to focus on searching for moons, this first planetary system discovered by HEK demonstrates the unexpected discoveries possible with transit analysis," said Kipping.

Source: Southwest Research Institute

Looking for Earths by looking for Jupiters

Montage of Jupiter and the Galilean satellites, Io, Europa, Ganymede, and Callisto.
Credit: NASA/Jet Propulsion Laboratory

In the search for Earth-like planets, it is helpful to look for clues and patterns that can help scientist narrow down the types of systems where potentially habitable planets are likely to be discovered. New research from a team including Carnegie's Alan Boss narrows down the search for Earth-like planets near Jupiter-like planets. Their work indicates that the early post-formation movements of hot-Jupiter planets probably disrupt the formation of Earth-like planets.

Their work is published the week of May 7 by Proceedings of the National Academy of Sciences.

The team, led by Jason Steffen of the Fermilab Center for particle Astrophysics, used data from NASA's Kepler mission to look at so-called "hot Jupiter" planets -- those roughly Jupiter-sized planets with orbital periods of about three days. If a Jupiter-like planet has been discovered by a slight dimming of brightness in the star it orbits as it passes between the star and Earth, it is then possible -- within certain parameters -- to determine whether the hot-Jupiter has any companion planets.

Of the 63 candidate hot Jupiter systems identified by Kepler, the research team did not find any evidence for nearby companion planets. There are several possible explanations. One is that there are no companion planets for any of these hot Jupiters. Another is that the companions are too small in either size or mass to be detected using these methods. Lastly it is possible that there are companion planets, but that the configuration of their orbits makes them undetectable using these methods.

However, when expanding the search to include systems with either Neptune-like planets (known as "hot Neptunes"), or "warm Jupiters" (Jupiter-sized planets with slightly larger orbits than hot Jupiters), the team found some potential companions. Of the 222 hot Neptunes, there were two with possible companions, and of the 31 warm Jupiters, there were three with possible companions.

"The implications of these findings are that systems with Earth-like planets formed differently than systems with hot Jupiters," Boss said. "Since we believe that hot Jupiters formed farther out, and then migrated inward toward their stars, the inward migration disrupted the formation of Earth-like planets. If our sun had a hot Jupiter, we would not be here."

Source: Carnegie Institution

Planet Mercury a result of early hit-and-run collisions

New simulations show that Mercury and other unusually metal-rich objects in the solar system may be relics left behind by hit-and-run collisions in the early solar system. Credit: NASA/JPL/Caltech

Planet Mercury's unusual metal-rich composition has been a longstanding puzzle in planetary science. According to a study published online in Nature Geoscience July 6, Mercury and other unusually metal-rich objects in the solar system may be relics left behind by collisions in the early solar system that built the other planets.

The origin of planet Mercury has been a difficult question in planetary science because its composition is very different from that of the other terrestrial planets and the moon. This small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet. Its iron core makes up about 65 percent of Mercury's total mass; Earth's core, by comparison, is just 32 percent of its mass.

How do we get Venus, Earth and Mars to be mostly "chondritic" (having a more-or-less Earth-like bulk composition) while Mercury is such an anomaly? For Arizona State University professor Erik Asphaug, understanding how such a planet accumulated from the dust, ice and gas in the early solar nebula is a key science question.

There have been a number of failed hypotheses for Mercury's formation. None of them until now has been able to explain how Mercury lost its mantle while retaining significant levels of volatiles (easily vaporized elements or compounds, such as water, lead and sulfur). Mercury has substantially more volatiles than the moon does, leading scientists to think its formation could have had nothing to do with a giant impact ripping off the mantle, which has been a common popular explanation.

To explain the mystery of Mercury's metal-rich composition, ASU's Asphaug and Andreas Reufer of the University of Bern have developed a new hypothesis involving hit-and-run collisions, where proto-Mercury loses half its mantle in a grazing blow into a larger planet (proto-Venus or proto-Earth). One or more hit-and-run collisions could have potentially stripped away proto-Mercury's mantle without an intense shock, leaving behind a mostly-iron body and satisfying a number of the major puzzles of planetary formation -- including the retention of volatiles -- in a process that can also explain the absence of shock features in many of the mantle-stripped meteorites.

Asphaug and Reufer have developed a statistical scenario for how planets merge and grow based on the common notion that Mars and Mercury are the last two relics of an original population of maybe 20 bodies that mostly accreted to form Venus and Earth. These last two planets lucked out.

"How did they luck out? Mars, by missing out on most of the action -- not colliding into any larger body since its formation -- and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete," explains Asphaug, who is a professor in ASU's School of Earth and Space Exploration. "It's like landing heads two or three times in a row -- lucky, but not crazy lucky. In fact, about one in 10 lucky."

By and large, dynamical modelers have rejected the notion that hit-and-run survivors can be important because they will eventually be accreted by the same larger body they originally ran into. Their argument is that it is very unlikely for a hit-and-run relic to survive this final accretion onto the target body.

"The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population. That is, the average unaccreted body will have been subject to more than one hit-and-run collision," explains Asphaug. "We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle."

According to Reufer, who performed the computer modeling for the study, "Giant collisions put the final touches on our planets. Only recently have we started to understand how profound and deep those final touches can be.

"The implication of the dynamical scenario explains, at long last, where the 'missing mantle' of Mercury is -- it's on Venus or the Earth, the hit-and-run targets that won the sweep-up," says Asphaug.

Disrupted formation

The duo's modelling has revealed a fundamental problem with an idea implicit to modern theories of planet formation: that protoplanets grow efficiently into ever larger bodies, merging whenever they collide.

Instead, disruption occurs even while the protoplanets are growing.

"Protoplanets do merge and grow, overall, because otherwise there would not be planets," says Asphaug. "But planet formation is actually a very messy, very lossy process, and when you take that into account, it's not at all surprising that the 'scraps,' like Mercury and Mars, and the asteroids are so diverse."

These simulations are of great relevance to meteoritics, which, just like Mercury's missing mantle, faces questions like: Where's all the stripped mantle rock that got removed from these early core-forming planetesimals? Where are the olivine meteorites that correspond to the dozens or hundreds of iron meteorite parent bodies?

"It's not missing -- it's inside the mantles of the planets, ultimately," explains Asphaug. "It got gobbled up by the larger growing planetary bodies in every hit-and-run series of encounters."

Source: Arizona State University

Because you can't eat just one: Star will swallow two planets

In this artist's conception, the doomed world Kepler-56b is being tidally shredded and consumed by its aging host star. New research shows that Kepler-56b will be engulfed by its star in about 130 million years, while its sibling Kepler-56c will be swallowed in 155 million years. This is the first time that two known exoplanets in a single system have a predicted "time of death." Credit: David A. Aguilar (CfA)

Two worlds orbiting a distant star are about to become a snack of cosmic proportions. Astronomers announced today that the planets Kepler-56b and Kepler-56c will be swallowed by their star in a short time by astronomical standards. Their ends will come in 130 million and 155 million years, respectively.

"As far as we know, this is the first time two known exoplanets in a single system have a predicted 'time of death,'" says lead author Gongjie Li of the Harvard-Smithsonian Center for Astrophysics (CfA).

She presented her research today at a meeting of the American Astronomical Society.

The Kepler-56 system provides a glimpse into the future of our solar system. In about five billion years our Sun will become a red giant star, swelling to immense proportions and engulfing Mercury and Venus.

The star Kepler-56 is becoming a red giant star as well. It already has ballooned out to four times the Sun's size. As it ages, it will continue to expand outward. Not only will the star grow larger, but its tides will get stronger, dragging its planets inward to their eventual doom.

Kepler-56b orbits its host star once every 10.5 days, while Kepler-56c orbits every 21.4 days. Both of them are much closer to their star than Mercury is to the Sun. As a result, they will meet their fate much faster. Li and her collaborators calculated the evolution of both the star's size (using the publicly available MESA code) and the planets' orbits to predict when the planets will be destroyed.

Even before they vanish, the two planets will be subjected to immense heating from the steadily growing star. Their atmospheres will begin to boil off, and the planets themselves will be stretched into egg shapes by stellar tides.

The only survivor in the system will be Kepler-56d, a gas giant planet circling in a 3.3-Earth-year orbit. It will watch from a safe distance as its two sibling worlds meet their demise.

The Kepler-56 planetary system also is notable for being the first "tilted" multiplanet system to be discovered. The orbits of the inner two planets are tipped significantly from the star's equator. This was unexpected since planets form from the same disk of gas and dust as the star, so they should orbit in nearly the same plane as the star's equator (as do the planets in our solar system).

The team was able to better constrain the tilt of these planets, compared to earlier work. 

They found that the most probable tilt was either 37 or 131 degrees.

Li and her colleagues also investigated the inclination of the outer planet and determined that its orbit is likely to be tilted relative to the star as well. Future observations should help astronomers to characterize this system, and to explain how it became so skewed.

Source: Harvard-Smithsonian Center for Astrophysics

Ancient volcanic explosions shed light on Mercury's origins

Measuring geological time: Two pyroclastic vents on the floor of Mercury’s Kipling crater, top, would likely not have survived the impact; they are more recent. The false color image of the same spot, bottom, marks pyroclastic material as brownish red. Credit: Image courtesy of Brown University

The surface of Mercury crackled with volcanic explosions for extended periods of the planet's history, according to a new analysis led by researchers at Brown University. The findings are surprising considering Mercury wasn't supposed to have explosive volcanism in the first place, and they could have implications for understanding how Mercury formed.

On Earth, volcanic explosions like the one that tore the lid off Mount St. Helens happen because our planet's interior is rich in volatiles -- water, carbon dioxide and other compounds with relatively low boiling points. As lava rises from the depths toward the surface, volatiles dissolved within it change phase from liquid to gas, expanding in the process. The pressure of that expansion can cause the crust above to burst like an overinflated balloon.

Mercury, however, was long thought to be bone dry when it comes to volatiles, and without volatiles there can't be explosive volcanism. But that view started to change in 2008, after NASA's MESSENGER spacecraft made its first flybys of Mercury. Those glimpses of the surface revealed deposits of pyroclastic ash -- the telltale signs of volcanic explosions -- peppering the planet's surface. It was a clue that at some point in its history Mercury's interior wasn't as bereft of volatiles as had been assumed.

What wasn't clear from those initial flybys was the timeframe over which those explosions occurred. Did Mercury's volatiles escape in a flurry of explosions early in the planet's history or has Mercury held on to its volatiles over a much longer period?

This latest work, available in online early view at the Journal of Geophysical Research: Planets, suggests the latter.

A team of researchers led by Tim Goudge, a graduate student in the Department of Geological Sciences at Brown, looked at 51 pyroclastic sites distributed across Mercury's surface. They used data from MESSENGER's cameras and spectrometers collected after the spacecraft entered orbit around Mercury in 2011. Compared with the data from the initial flybys, the orbital data provided a much more detailed view of the deposits and the source vents that spat them out.

The new MESSENGER data revealed that some of the vents have eroded to a much greater degree than others -- an indicator that the explosions didn't happen all at the same time.

"If [the explosions] happened over a brief period and then stopped, you'd expect all the vents to be degraded by approximately the same amount," Goudge said. "We don't see that; we see different degradation states. So the eruptions appear to have been taking place over an appreciable period of Mercury's history."

But just where that period of explosiveness fits into Mercury's geological history was another matter. To help figure that out, Goudge and his colleagues took advantage of the fact that most of the sites are located within impact craters. The age of each crater offers an important constraint in the age of the pyroclastic deposit inside it: The deposit has to be younger than its host crater. If the deposit had come first, it would have been obliterated by the impact that formed the crater. So the age of the crater provides an upper limit on how old the pyroclastic deposit can be.

As it happens, there's an established method for dating craters on Mercury. The rims and walls of craters become eroded and degraded over time, and the extent of that degradation can be used to get an approximate age of the crater.

Using that method, Goudge and his colleagues showed that some pyroclastic deposits are found in relatively young (geologically speaking) craters dated to between 3.5 and 1 billion years old. The finding helps rule out the possibility that all the pyroclastic activity happened shortly after Mercury's formation around 4.5 billion years ago.

"These ages tell us that Mercury didn't degas all of its volatiles very early," Goudge said. "It kept some of its volatiles around to more recent geological times."

The extent to which Mercury's volatiles stuck around could shed light on how the planet formed. Despite being the smallest planet in the solar system (since Pluto was demoted from the ranks of the planets), Mercury has an abnormally large iron core. That finding led to speculation the perhaps Mercury was once much larger, but had its outer layers removed -- either fried away by the nearby Sun or perhaps blasted away be a huge impact early in the planet's history. Either of those events, however, would likely have heated the outer parts of Mercury enough to remove volatiles very early in its history.

In light of this study and other data collected by MESSENGER showing traces of the volatiles sulfur, potassium, and sodium on Mercury's surface, both those scenarios seem increasingly unlikely.

"Together with other results that suggest the Moon may have had more volatiles than previously thought, this research is revolutionizing our thinking about the early history of the planets and satellites," said Jim Head, professor of geological sciences and a MESSENGER mission co-investigator. "These results define specific targets for future exploration of Mercury by orbiting and landed spacecraft."

Source: Brown University

Stretched-out solid exoplanets

An artist’s impression of a stretched rocky planet in orbit around a red dwarf star. So close to the star, there is a difference in the strength of the gravitational field on each side of the planet, stretching it significantly.
Credit: Shivam Sikroria

Astronomers could soon be able to find rocky planets stretched out by the gravity of the stars they orbit, according to a group of researchers in the United States. The team, led by Prabal Saxena of George Mason University, describe how to detect these exotic worlds in a paper in the journal Monthly Notices of the Royal Astronomical Society.

Since the first discovery in 1993, more than 1800 planets have been found in orbit around stars other than our Sun. These 'exoplanets' are incredibly diverse, with some gaseous like Jupiter and some mostly rocky like the Earth. The worlds also orbit their stars at very different distances, from less than a million km to nearly 100 billion km away. Planets that are very close to their stars experience harsh conditions, often with very high temperatures (>1000 degrees Celsius) and significant stretching from the tidal forces resulting from the stellar gravitational field. This is most obvious with planets with a large atmosphere (so-called 'hot Jupiters') but harder to see with the rockier objects.

Prabal and his team modelled cases where the planets are in orbit close to small red dwarf stars, much fainter than our Sun, but by far the most common type of star in the Galaxy. The planets' rotation is locked, so the worlds keep the same face towards the stars they orbit, much like the Moon does as it moves around the Earth. According to the scientists, in these circumstances the distortion of the planets should be detectable in transit events, where the planets moves in front of their stars and blocks out some of their light.

If astronomers are able to find these extreme exoplanets, it could give them new insights into the properties of Earth-like planets as a whole. Prabal comments, "Imagine taking a planet like the Earth or Mars, placing it near a cool red star and stretching it out. Analysing the new shape alone will tell us a lot about the otherwise impossible to see internal structure of the planet and how it changes over time."

The subtle signals from stretched rocky planets could be found by some current telescopes, and certainly by much more powerful observatories like the James Webb Space Telescope (JWST) and the European Extremely Large Telescope (E-ELT) that are due to enter service in the next few years.

Source: Royal Astronomical Society (RAS)

Dwarf planet Haumea shines with crystalline ice

The tiny and strange planet Haumea moves beyond the orbit of Neptune. Credit: SINC/José Antonio Peñas

The fifth dwarf planet of the solar system, Haumea, and at least one of its two satellites, are covered in crystalline water-ice due to the tidal forces between them and the heat of radiogenic elements. This is the finding of an international research study using observations from the VLT telescope at the European Southern Observatory in Chile.

The tiny and strange planet Haumea moves beyond the orbit of Neptune. It has the shape of a flattened rugby ball and is around 2,000 km long. It spins completely in less than four hours, at one of the fastest rotation speeds in the solar system. The crystallised water that covers this planet and its two satellites (Hi'iaka and Namaka) makes them shine in the darkness of space.

Now an international research team has confirmed that 75% of Haumea and 100% of Hi'iaka (which is around 400 km in diameter) are covered with crystallised water-ice (with an ordered structure) and not, as would have been expected, with amorphous ice disorganised due to solar radiation. The study suggests that the planet is made up of a frozen outer layer and an internal section made up of between 88% and 97% rock (with a density of 3.5 g/cm3).

"Since solar radiation constantly destroys the crystalline structure of ice on the surface, energy sources are required to keep it organised. The two that we have taken into consideration are that able to generate radiogenic elements (potassium-40, thorium-232 and uranium-238) from the inside, and the tidal forces between Haumea and its satellites (as seen between Earth and the Moon)," Benoit Carry, co-author of the study and a researcher at the ESAC Centre of the European Space Agency (ESA) in Madrid (Spain), said.

The researcher also highlights other peculiarities of Haumea: "Its orbital plane is inclined at 28º with respect to the usual plane of planets in the solar system, the orbits of its satellites are not on the same plane either -- which is very unusual -- and the entire system belongs to a single family within the frozen objects in the Kuiper Belt (at a distance of between 4.5 billion and more than 15 billion kilometres from the Sun)."

According to the scientists, the two satellites could have been created by another object smashing into Haumea, which could also have originated the rapid rotation of the dwarf planet (3.9 hours) and have moulded it into its rugby ball shape. Some numerical models have demonstrated that this could be caused by a fairly tangential impact.

Observations from the SINFONI instrument of the Very Large Telescope (VLT), the enormous telescope of the European Southern Observatory (ESO) in Chile, were used in order to carry out the study, which has been published in the journal Astronomy & Astrophysics. ESO astronomer Christophe Dumas led this study from the observatory.
"SINFONI is an integral field spectrometer that provides 'data cubes' in which two of the dimensions are spatial (like those of any flat image), while the third is spectral, meaning that each layer of the cube is an image taken with a different wave size," explains Carry.

The mystery of Haumea

The scientist acknowledges that the precise orbits and sizes of the dwarf planet are still not known (they are operating with approximate scales of around 2,000 x 1,500 x 1,000 km) nor are those of its satellites. In reality, these are two very distant bright points of light, the data for which are obtained indirectly.

In the case of the tiny Namaka (around 200 km in diameter), the signal at the time it was observed was so weak that it was impossible to obtain information about its surface, although new data on its orbit were gathered. Meanwhile, the models for the tidal forces of this strange system are also improving.

Another of the mysteries of Haumea is the presence of a dark, reddish spot, which contrasts with the whitish colour of the planet. "My interpretation of the infrared photometry is that this area could be a richer source of crystalline water-ice than the rest of the surface," Pedro Lacerda, co-discoverer of the spot and an astronomer at Queen's University in Belfast (United Kingdom), said. The researcher does not rule out the possibility of some kind of irradiated mineral or organic matter having caused this colouration.

Haumea is the fifth dwarf planet in the solar system along with Pluto, Ceres, Eris and Makemake. Its existence was confirmed in 2005, when it was called 2003 EL61 (from the international nomenclature code: year of first observation, half and order number).

Two teams of astronomers contested the discovery. The first group was led by the Spanish researcher José Luis Ortiz Moreno from the Institute of Astrophysics of Andalusia (CSIC), while the other was led by the astrophysicist Michael E. Brown from the California Institute of Technology (Caltech, USA).

In the end, the International Astronomical Union decided to accept the discovery by the Spanish team, but named the strange dwarf planet and its satellites according to names suggested by the American team. In Hawaiian mythology, Haumea is the goddess of fertility and birth, and Hi'iaka and Namaka are two of her daughters.

Source: Plataforma SINC

Astronomy: Debris-strewn exoplanetary construction yards

This is a set of images from a NASA Hubble Space Telescope survey of the architecture of debris systems around young stars. Ten previously discovered circumstellar debris systems, plus MP Mus (a mature protoplanetary disk of age comparable to the youngest of the debris disks), were studied. Hubble's sharp view uncovers an unexpected diversity and complexity in the structures. The disk-like structures are vast, many times larger than the planetary distribution in our solar system. Some disks are tilted edge-on to our view, others nearly face-on. Asymmetries and warping in the disks might be caused by the host star's passage though interstellar space. Alternatively, the disks may be affected by the action of unseen planets. In particular, the asymmetry in HD 181327 looks like a spray of material that is very distant from its host star. It might be the aftermath of a collision between two small bodies, suggesting that the unseen planetary system may be chaotic. The stars surveyed may be as young as 10 million years old and as mature as more than 1 billion years old. The visible-light survey was done with the Space Telescope Imaging Spectrograph (STIS). The STIS coronagraph blocks out the light from the host star so that the very faint reflected light from the dust structures can be seen. The images have been artificially colored to enhance detail.Credit: NASA, ESA, G. Schneider (University of Arizona), and the HST/GO 12228 Team

Astronomers using NASA's Hubble Space Telescope have completed the largest and most sensitive visible-light imaging survey of dusty debris disks around other stars. These dusty disks, likely created by collisions between leftover objects from planet formation, were imaged around stars as young as 10 million years old and as mature as more than 1 billion years old.

"It's like looking back in time to see the kinds of destructive events that once routinely happened in our solar system after the planets formed," said survey leader Glenn Schneider of the University of Arizona's Steward Observatory. The survey's results appeared in the Oct. 1, 2014, issue of The Astronomical Journal.

Once thought to be simply pancake-like structures, the unexpected diversity and complexity and varying distribution of dust among these debris systems strongly suggest these disks are gravitationally affected by unseen planets orbiting the star. Alternatively, these effects could result from the stars' passing through interstellar space.

The researchers discovered that no two "disks" of material surrounding stars look the same. "We find that the systems are not simply flat with uniform surfaces," Schneider said. "These are actually pretty complicated three-dimensional debris systems, often with embedded smaller structures. Some of the substructures could be signposts of unseen planets." The astronomers used Hubble's Space Telescope Imaging Spectrograph to study 10 previously discovered circumstellar debris systems, plus comparatively, MP Mus, a mature protoplanetary disk of age comparable to the youngest of the debris disks.

Irregularities observed in one ring-like system in particular, around a star called HD 181327, resemble the ejection of a huge spray of debris into the outer part of the system from the recent collision of two bodies.

"This spray of material is fairly distant from its host star -- roughly twice the distance that Pluto is from the Sun," said co-investigator Christopher Stark of NASA's Goddard Space Flight Center, Greenbelt, Maryland. "Catastrophically destroying an object that massive at such a large distance is difficult to explain, and it should be very rare. If we are in fact seeing the recent aftermath of a massive collision, the unseen planetary system may be quite chaotic."

Another interpretation for the irregularities is that the disk has been mysteriously warped by the star's passage through interstellar space, directly interacting with unseen interstellar material. "Either way, the answer is exciting," Schneider said. "Our team is currently analyzing follow-up observations that will help reveal the true cause of the irregularity."

Over the past few years astronomers have found an incredible diversity in the architecture of exoplanetary systems -- planets are arranged in orbits that are markedly different than found in our solar system. "We are now seeing a similar diversity in the architecture of accompanying debris systems," Schneider said. "How are the planets affecting the disks, and how are the disks affecting the planets? There is some sort of interdependence between a planet and the accompanying debris that might affect the evolution of these exoplanetary debris systems."

From this small sample, the most important message to take away is one of diversity, Schneider said. He added that astronomers really need to understand the internal and external influences on these systems, such as stellar winds and interactions with clouds of interstellar material, and how they are influenced by the mass and age of the parent star, and the abundance of heavier elements needed to build planets.

Though astronomers have found nearly 4,000 exoplanet candidates since 1995, mostly by indirect detection methods, only about two dozen light-scattering, circumstellar debris systems have been imaged over that same time period. That's because the disks are typically 100,000 times fainter than, and often very close to, their bright parent stars. The majority have been seen because of Hubble's ability to perform high-contrast imaging, in which the overwhelming light from the star is blocked to reveal the faint disk that surrounds the star.

The new imaging survey also yields insight into how our solar system formed and evolved 4.6 billion years ago. In particular, the suspected planet collision seen in the disk around HD 181327 may be similar to how the Earth-Moon system formed, as well as the Pluto-Charon system over 4 billion years ago. In those cases, collisions between planet-sized bodies cast debris that then coalesced into a companion moon.

Source: Space Telescope Science Institute (STScI)