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