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

How to estimate the magnetic field of an exoplanet

Artist's interpretation of Planet HD 209458b. Scientists have now estimated the value of the magnetic moment of the planet HD 209458b. Credit: NASA/ESA/CNRS/Alfred Vidal-Madjar

Scientists developed a new method which allows to estimate the magnetic field of a distant exoplanet, i.e., a planet, which is located outside the Solar system and orbits a different star. Moreover, they managed to estimate the value of the magnetic moment of the planet HD 209458b.The group of scientists including one of the researchers of the Lomonosov Moscow State University (Russia) published their article in the Science magazine.

In the two decades which passed since the discovery of the first planet outside the Solar system, astronomers have made a great progress in the study of these objects. While 20 years ago a big event was even the discovery of a new planet, nowadays astronomers are able to consider their moons, atmosphere and climate and other characteristics similar to the ones of the planets in the Solar system. One of the important properties of both solid and gaseous planets is their possible magnetic field and its magnitude. On Earth it protects all the living creatures from the dangerous cosmic rays and helps animals to navigate in space.

Kristina Kislyakova of the Space Research Institute of the Austrian Academy of Sciences in Graz together with an international group of physicists for the first time ever was able to estimate the value of the magnetic moment and the shape of the magnetosphere of the exoplanet HD 209458b. Maxim Khodachenko, a researcher at the Department of Radiation and computational methods of the Skobeltsyn Institute of Nuclear Physics of the Lomonosov Moscow State University, is also one of the authors of the article. He also works at the Space Research Institute of the Austrian Academy of Sciences.

Planet HD 209458b (Osiris) is a hot Jupiter, approximately one third larger and lighter than Jupiter. It is a hot gaseous giant orbiting very close to the host star HD 209458. HD 209458b accomplishes one revolution around the host star for only 3.5 Earth days. It has been known to astronomers for a long time and is relatively well studied. In particular, it is the first planet where the atmosphere was detected. Therefore, for many scientists it has become a model object for the development of their hypotheses.

Scientists used the observations of the Hubble Space Telescope of the HD 209458b in the hydrogen Lyman-alpha line at the time of transit, when the planet crosses the stellar disc as seen from Earth. At first, the scientists studied the absorption of the star radiation by the atmosphere of the planet. Afterwards they were able to estimate the shape of the gas cloud surrounding the hot Jupiter, and, based on these results, the size and the configuration of the magnetosphere.

"We modeled the formation of the cloud of hot hydrogen around the planet and showed that only one configuration, which corresponds to specific values of the magnetic moment and the parameters of the stellar wind, allowed us to reproduce the observations," explained Kristina Kislyakova.

To make the model more accurate, scientists accounted for many factors that define the interaction between the stellar wind and the atmosphere of the planet: so-called charge exchange between the stellar wind and the neutral atmospheric particles and their ionization, gravitational effects, pressure, radiation acceleration, and the spectral line broadening.

At present, scientists believe that the size of the atomic hydrogen envelope is defined by the interaction between the gas outflows from the planet and the incoming stellar wind protons. Similarly to Earth, the interaction of the atmosphere with the stellar wind occurs above the magnetosphere. By knowing the parameters of an atomic hydrogen cloud, one can estimate the size of the magnetosphere by means of a specific model.

Since direct measurements of the magnetic field of exoplanets are currently impossible, the indirect methods are broadly used, for example, using the radio observations. There exist a number of attempts to detect the radio emission from the planet HD 209458b. However, because of the large distances the attempts to detect the radio emission from exoplanets have yet been unsuccessful.

"The planet's magnetosphere was relatively small beeing only 2.9 planetary radii corresponding to a magnetic moment of only 10% of the magnetic moment of Jupiter," explained Kislyakova, a graduate of the Lobachevsky State University of Nizhny Novgorod. According to her, it is consistent with the estimates of the effectiveness of the planetary dynamo for this planet.

"This method can be used for every planet, including Earth-like planets, if there exist an extended high energetic hydrogen envelope around them," summarized Maxim Khodachenko.

Source: Lomonosov Moscow State University

Planet forming around star about 335 light years from Earth

An artist's conception of the young massive star HD100546 and its surrounding disk. A planet forming in the disk has cleared the disk within 13AU of the star, a distance comparable to that of Saturn from the sun. As gas and dust flows from the circumstellar disk to the planet, this material surrounds the planet as a circumplanetary disk (inset). These rotating disks are believed to be the birthplaces of planetary moons, such as the Galilean moons that orbit Jupiter. While they are theoretically predicted to surround giant planets at birth, there has been little observational evidence to date for circumplanetary disks outside the solar system. Brittain et al. (2014) report evidence for an orbiting source of carbon monoxide emission whose size is consistent with theoretical predictions for a circumplanetary disk. Observations over 10 years trace the orbit of the forming planet from behind the near side of the circumstellar disk in 2003 to the far side of the disk in 2013. These observations provide a new way to study how planets form. Credit: P. Marenfeld & NOAO/AURA/NSF

Dr. John Carr, a scientist at the U.S. Naval Research Laboratory (NRL), is part of an international team that has discovered what they believe is evidence of a planet forming around a star about 335 light years from Earth. This research is published in the August 20th issue of The Astrophysical Journal.

Carr and the other research team members set out to study the protoplanetary disk around a star known as HD 100546, and as sometimes happens in scientific inquiry, it was by "chance" that they stumbled upon the formation of the planet orbiting this star. A protoplanetary disk, or circumstellar disk, is a very large disk of material orbiting a newly formed star out of which a planetary system may form. The team was studying the warm gas in this disk using a technique called spectro-astrometry, which allows astronomers to detect small changes in the position of moving gas.

The researchers discovered an "extra" source of gaseous emission from carbon monoxide molecules that could not be explained by the protoplanetary disk alone. By tracking the changes in velocity and position of this extra emission over the years of the observations, they were able to show that it is orbiting around the young star. The distance from the star is somewhat larger than the distance of Saturn from the Sun. The evidence suggests that they are observing hot gas that surrounds an orbiting young planet. Carr points out that rather than seeing the planet directly, they are detecting the gas as it swirls around and onto the forming planet.

Through modeling carried out by Dr. Sean Brittain, a Clemson University astrophysicist and the lead author on the paper, and with additional data gathered by the team to confirm their initial hypothesis, they were able to investigate the extra emission as it orbited the star. The authors concluded that a likely explanation for the observations is a small circumplanetary disk of hot gas orbiting a forming planet. The candidate planet would be a gas giant at least three times the mass of Jupiter. The theory is that material from the large protoplanetary disk feeds into the circumplanetary disk, which then feeds onto the growing planet. Hence, a circumplanetary disk plays a mediating role in the growth of the planet. The remnants of a circumplanetary disk could also give birth to moons, such as those seen around Jupiter in our solar system. As Carr explains, a novel aspect of this new evidence for planet formation is the possible detection of a circumplanetary disk.

The team's study is based on four sets of observations gathered in 2003, 2006, 2010, and 2013. They used the Gemini Observatory and the Very Large Telescope at the European Southern Observatory, both located in Chile. The Gemini Observatory consists of twin 8.1-meter diameter optical/infrared telescopes located on mountains in Hawaii and Chile. The VLT is not just one telescope, but an array of four, each with a main mirror of 8.2 meters in diameter. The data were collected using high-resolution infrared spectrographs that allowed precise measurements of the motions of molecular gas surrounding the star.

"These results provide a rare opportunity," Carr says, "to study planet formation in action. Our analysis strongly suggests we are observing a disk of hot gas that surrounds a forming giant planet in orbit around the star. While such circumplanetary disks have been theorized to surround giant planets at birth and to control the flow of gas onto the growing planet, these findings are the first observational evidence for their existence. If our interpretation is correct, we are essentially seeing a planet caught in the act of formation."

Looking ahead, the team would like to continue to monitor the motion of the planet and obtain additional data to better define the properties of the circumplanetary disk. They predict that the planet and its disk will disappear from view in about two years time when they become hidden by the inner edge of the circumstellar disk. So, if the team's model is correct, the signature of the orbiting planet will not be seen for another 15 years until its orbit brings it back into view.

Source: Naval Research Laboratory

Artist impression of the debris disc and planets around the star known as Gliese 581, superimposed on Herschel PACS images at 70, 100 and 160 micrometre wavelengths. The line drawing superimposed on the Herschel image gives a schematic representation of the location and orientation of the star, planets and disc, albeit not to scale. Credit: Image courtesy of European Space Agency (ESA)

Using ESA's Herschel space observatory, astronomers have discovered vast comet belts surrounding two nearby planetary systems known to host only Earth-to-Neptune-mass worlds. The comet reservoirs could have delivered life-giving oceans to the innermost planets.

In a previous Herschel study, scientists found that the dusty belt surrounding nearby star Fomalhaut must be maintained by collisions between comets.

In the new Herschel study, two more nearby planetary systems -- GJ 581 and 61 Vir -- have been found to host vast amounts of cometary debris.

Herschel detected the signatures of cold dust at 200ºC below freezing, in quantities that mean these systems must have at least 10 times more comets than in our own Solar System's Kuiper Belt.

GJ 581, or Gliese 581, is a low-mass M dwarf star, the most common type of star in the Galaxy. Earlier studies have shown that it hosts at least four planets, including one that resides in the 'Goldilocks Zone' -- the distance from the central sun where liquid surface water could exist.

Two planets are confirmed around G-type star 61 Vir, which is just a little less massive than our Sun.

The planets in both systems are known as 'super-Earths', covering a range of masses between 2 and 18 times that of Earth.

Interestingly, however, there is no evidence for giant Jupiter- or Saturn-mass planets in either system.

The gravitational interplay between Jupiter and Saturn in our own Solar System is thought to have been responsible for disrupting a once highly populated Kuiper Belt, sending a deluge of comets towards the inner planets in a cataclysmic event that lasted several million years.

"The new observations are giving us a clue: they're saying that in the Solar System we have giant planets and a relatively sparse Kuiper Belt, but systems with only low-mass planets often have much denser Kuiper belts," says Dr Mark Wyatt from the University of Cambridge, lead author of the paper focusing on the debris disc around 61 Vir.

"We think that may be because the absence of a Jupiter in the low-mass planet systems allows them to avoid a dramatic heavy bombardment event, and instead experience a gradual rain of comets over billions of years."

"For an older star like GJ 581, which is at least two billion years old, enough time has elapsed for such a gradual rain of comets to deliver a sizable amount of water to the innermost planets, which is of particular importance for the planet residing in the star's habitable zone," adds Dr Jean-Francois Lestrade of the Observatoire de Paris who led the work on GJ 581.

However, in order to produce the vast amount of dust seen by Herschel, collisions between the comets are needed, which could be triggered by a Neptune-sized planet residing close to the disc.

"Simulations show us that the known close-in planets in each of these systems cannot do the job, but a similarly-sized planet located much further from the star -- currently beyond the reach of current detection campaigns -- would be able to stir the disc to make it dusty and observable," says Dr Lestrade.

"Herschel is finding a correlation between the presence of massive debris discs and planetary systems with no Jupiter-class planets, which offers a clue to our understanding of how planetary systems form and evolve," says Göran Pilbratt, ESA's Herschel project scientist.

Source: European Space Agency (ESA)

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