Mysterious ultra-dense planets spotted by Kepler telescope

Ultra-dense planets may be the 'fossil cores' of ice giants stripped of their outer layers by migrating too close …
Astronomers have spotted strange ultra-dense objects orbiting around other stars, which they believe could be a new type of planet they've never seen before.

Since the Kepler telescope started its mission back in 2009, it has detected an amazingly diverse population of exoplanets in just the small section of the galaxy that it has been viewing. The telescope finds planets by watching for the characteristic 'dip' they cause in their star's light as they pass in front of it, and by examining these planets further, astronomers can figure out their size and mass.

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In many cases, the planets found vary quite a bit in size, but they still fall into the same categories we see in our own solar system — terrestrial like Earth and the other inner planets, gas giants like Jupiter and Saturn, and ice giants like Neptune and Uranus. However, there are some cases that are coming to light now that are unlike anything that's been seen so far — planets the size of Earth, but whose mass is much higher than expected, giving them a density that's greater than that of pure iron.

"There is no way to explain that in the Solar System," said Olivier Grasset, Deputy Director of the Laboratory of Planetology at the University of Nantes in France, according to Nature.

However, Grasset and his colleagues have an idea about what these objects might be — 'fossil' remnants of ice giants, like Neptune and Uranus, that lost their outer layers as they wandered too close to their star.

It is well known that as planets form from the disk of dust and debris around a star, they tend to migrate inward, towards their star, as they grow in size and are slowed down by collisions and drag due to the matter in the disk. Eventually they reach a point where they sweep up enough of the disk that their mass and speed level off and they settle into a stable orbit. This can be far out, like the giant planets of our Solar System, or they can be so close in that even planets several times the mass of Jupiter whip around their star in just a matter of days or hours.

If a large gas giant or ice giant slowly migrates in close to their star, the outer layers of the planet — its atmosphere of hydrogen, helium and water — heats up so much that it gets boiled off. If this happens slowly, over millions of years, the lower 'super-compressed', super-heated layers of the planet have the time to 'relax', expanding to replace the boiled off layers and the planet retains its basic structure.

However, Grasset and the others believe that if this process were to happen very quickly, over thousands of years instead of millions, the much more rapid cooling of the inner layers of the planet could lock them in their hyper-compressed state permanently.

"If the process is short, you end up with a very compressed super-Earth," said Grasset.

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This is, of course, speculation right now.

Science is still working to better understand what goes on inside the cores of ice-giant planets, and how the core of one of these planets would respond to being stripped naked, but Lars Stixrude, a geologist at University College London, does call the idea "fascinating."

According to Kepler mission leader William Borucki, the idea is plausible, however there are other ways an ice giant could lose its outer layers — such as from a collision — or it could be that these planets formed, as they are, under entirely different circumstances, perhaps similar to how stars form. In any case, he finds it exciting that the Kepler mission is challenging old assumptions.

"This is why we do science."

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