Kepler-10c: Earth's older, bigger cousin

C Sivaram explores the significance of the discovery of Kepler-10c, a new rocky planet

Kepler-10c: Earth's older, bigger cousin

The Kepler telescope, during its working lifespan of a few years discovered hundreds of exoplanets (extrasolar planet), many of which are ‘Earth-like’. Recently, a new planet, Kepler-10c, part of the Kepler-10 system, was found.

Kepler-10c is a Sun-like star. The newly found planet circles this star once every 45 days. This is about half the orbital period of Mercury, the planet in the solar system, with an orbital duration of 88 days. 

For a Sun-like host star, this implies that the newly-found planet orbits its parent star at a distance nearly five times lesser than the Earth-Sun distance.

This in turn implies that it receives a radiation flux from its host star, twenty times the solar flux on Earth. With 20 kilowatts falling on each square metre, the planet would be scorching hot with a surface temperature of well over 700 degrees. 

However, what is remarkable about this planet is that although it is Earth-like, it is much heavier than the Earth, with a mass 17 times more. This was estimated using the HARPS-North instrument at the Galileo telescope in the Canary Islands. Astronomer Xavier Dumusque of the Harvard Smithsonian Centre was among those describing it as a ‘mega earth,’ much bigger than the previously discovered ‘super-earth’. Indeed, it has been described as a Godzilla planet after the popular movie monster of that name.  It has a diameter of about 30,000 km, more than twice that of the Earth. 

Implications

With its estimated mass, its density would turn out to be about twice that of the Earth, i.e., the density of Lead!  This would mean that the planet is all solid, rocky and indeed a super hot ball of ‘lead!’ As many planets in this mass range are Neptune-like, (so called hot Neptunes orbiting quite close to their stars), with low density, (i.e. they are gas giants), this all metallic rocky mega-Earth should surely count as an oddball among the newly discovered exoplanets.

If this dense planet had formed much further away from its host star, it could have accreted a lot of gaseous matter (methane, ammonia etc) to become a gas giant like Jupiter or Saturn, which are believed to have rocky cores of this mass.

A rocky planet with at least five earth masses, (this is considered the dividing mass line between dense terrestrial planets like Earth and the gas giants of the out solar system) could if formed in the much cooler outer regions far away from the host star, accrete over a period of time enough gas and dust to become a gas giant. 

As this massive rocky planet probably formed very close to its host star, all the volatiles and gases (including small dust particles) would not be present as they would have been driven away by the star’s radiation to the outer regions of the stellar system. So it would have remained a dense rocky massive planet rather than evolve into a gas giant with an average density just above that of water, like Jupiter!

In our own solar system, this is the reason why the inner terrestrial planets are much denser as the intense radiation would have swept away all the gases and volatile matter to the out regions (far away from the star) where the gas giants can form by accreting all this available matter onto their rocky cores!

Remarkable fact

However, what is really remarkable about the Kepler-10 system is that it is much older than our solar system. It is estimated to be about 11 billion years old, which means that it was formed less than three billion years after the universe started expanding from a dense hot phase.

It was earlier thought that rocky planets consisting of metals (and heavier elements) which are crucial for biological systems formed much later, belong to a later generation of stars. So the presence of a dense planet (full of metals) so soon after the universe evolved is a bit of a surprise and could revise our ideas of the origin of the earliest possible epochs when biological life could have arisen. It is thought that about six billion years are required for dense planets (having heavier elements) to proliferate facilitating the origin of life. 

Now it is known that the oldest stars formed hardly a few hundred million years after the universe began expanding and could be 13 billion years old. This is the first time that a dense planet associated with such old stellar system has been discovered. Apart from pushing back the possibility of life originating at a much earlier epoch, (in the evolution of the universe) it raises several questions about the origin of planets in general. 

This is a subject which still presents many unresolved conundrums and puzzles. Hopefully, the discovery of even older planetary systems in the next few years could resolve key issues. 

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