Mighty planets can be cut off, leaving only their rocky cores and becoming nothing larger than Superearths. While astronomers had long suspected this could happen, a new study shows it could happen in just a billion years.
The largest planets in the solar system – and indeed the galaxy – are mostly gas and ice: hydrogen, helium, water, ammonia, and so on. However, you likely have rocky cores buried deep beneath all of these suffocating layers.
And those layers can essentially stay there forever unless something blows them away.
Astronomers had long thought that a giant planet (e.g. something in the “Sub-Neptune” class of nearly Neptune-sized worlds) would receive enough radiation from its parent star, eventually lose its atmosphere, leaving only its rocky core behind (in this case a so-called Superearth). But they didn’t know how long this process might take – there might just not be enough time in the entire history of the universe for this process.
However, the combination of two surveys offers one possible answer. The first poll is the Kepler Mission, which saw thousands of exoplanets orbiting other worlds, including Subneptune and Superearths.
The second survey is the Gaia Mission, which mapped the same stars (and many, many more) and made precise measurements of the star’s properties that can be used to estimate their ages.
The result is a new study that appears in the Astronomical Journal. Of the subset of stars that host large planets in their vicinity (where the average amount of starlight can be up to 150 times that of Earth), older stars tend to host smaller worlds. The younger the star is on average, the more likely it is that it has a larger subneptune. The older the star, the more likely it is that it has a smaller aboveground.
“While astronomers have long predicted that planets should shrink as they age, we didn’t know if this could happen over billions of years. Let’s do that now, ”says Travis Berger, PhD student in astronomy at the University of Hawaii, who led the study. “The fact that planet sizes change on billion-year timescales suggests that there is an evolutionary path where brightly lit sub-Neptune-sized planets transition to super-Earth-sized planets.”
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