While the sizes and places that planets form make sense, the next bit in their evolution gets a bit weird. In looking at planets going around stars, we see different sizes of planets around young stars than around old stars. This doesn’t seem to be a “stars used to make smaller planets” problem. This seems to be “planets, unlike people, lose mass as they enter middle age”.

In general, it’s been known since 2017 that there is a lack of planets between 1.5 and 2 times the size of the Earth. In a new study published in The Astronomical Journal and led by Trevor David, researchers looked at 1,400 planets whose host stars had ages between 500 million and twelve billion years. These ages weren’t known to a high degree of accuracy, so researchers looked at planets belonging to stars younger than a couple of billion years and stars older than 2-3 years.

Side note: our solar system counts as old for the purposes of this study.

Looking at these two bins of young and old stars, with their correspondingly younger and older planets, they found that planets in older systems tended to be 5% smaller on average. Along the way, planets actually change what kind of a planet they are. Ice giants like Neptune, which is roughly four times Earth’s radius, seem to lose their massive atmospheres and leave behind terrestrial worlds that are much much closer in size to Earth. Exactly how this happens is something still being determined. It could be related to the stellar activity that boils off atmospheres, the cooling of planetary cores that causes magnetic fields to shut down and atmospheres to be lost in normal stellar winds, or something entirely different or all of the above.

What intrigues me the most about this research is it seems to hint that all these massive hot Jupiters we’re seeing next to their stars could one day become what we call terrestrial worlds. It also makes me wonder: just how did Mercury start? And how will Jupiter and other gas giants end when one day our Sun bloats up and blasts our solar system with its shedding atmosphere?

More Information

Gap in Exoplanet Size Shifts with Age (Eos)

“Evolution of the Exoplanet Size Distribution: Forming Large Super-Earths Over Billions of Years,” Trevor J. David et al., 2021 May 14, The Astronomical Journal