The mystery of Super Earth and mini-Neptune can finally be solved

​​​​​​Scientists have discovered that some planets can migrate into the core of their planetary system early in their lives, possibly explaining the lack of planets we see that are about twice the width of Earth.

Over the years, scientists have managed to observe many exoplanets that are smaller or larger than Earth, but planets that are exactly 1.6 to 2.2 times the size of our Earth are relatively rare. In particular, exoplanets defined as super-Earths or mini-Neptunes seem to be missing from space. They are classified as such if they are slightly more than twice the size of our planet, but still smaller than the ice giant Neptune.

The absence of these planets is therefore known as the “valley ray” or “gap ray”, and it has troubled scientists for a long time.

But now, new research suggests that the “missing” super-Earths and mini-Neptunes may have taken different routes out of the radial valley.

“Six years ago, a reanalysis of data from the Kepler space telescope revealed a dearth of exoplanets with sizes around two Earth radii,” Remo Burn, an exoplanet expert at the Max Planck Institute for Astronomy, said in a statement.

Related: Surprise! Children’s exoplanets may look like Smarties candies rather than spheres

Scientists have known for many years that planets can move towards or away from their parent stars after formation, but only how effective this migration would be to the radial valley was previously known. create.

The most common explanation for the valley, according to Burn, is related to the stars irradiating the planets that surround them, removing the atmosphere of that universe and contracting them. This theory alone, however, was not satisfactory to him. “This explanation neglects the influence of planetary migration,” he explained.

So Burn led a team of researchers who tried to investigate whether planetary migration could supplement the standard explanation and provide an additional explanation for why so few super-Earths and mini-Neptunes are seen orbiting close to their stars.

Super-Earth shrinks and sub-Neptune grows

Because the two planets that occupy the radius gap, super-Earth and mini-Neptune, are both absent from the solar system, scientists cannot fully study both sides. Howeer, researchers are fairly certain that super-Earths are rocky or terrestrial planets, although the characteristics of mini-Neptune are less certain.

What scientists also agree on is that mini-Neptune, also known as sub-Neptune, should have an atmosphere that extends far beyond the rocky planets.

Burn and the team wanted to know if this fact could have played a role in the creation of the ray valley, and if it could indicate very different formations and evolutions for the super-Earths and mini-Neptunes because that valley there.

Reanalyzing a simulation the team ran in 2020, Burn and his colleagues looked at processes in the gas and dust disks surrounding young stars that give rise to new planets, the formation of atmospheres, and transit of planets.

Understanding how water behaves over a wide range of pressures and temperatures was critical to the simulation and developing a potential solution for the radial valley — which also factors in planetary migration. This is because these parameters allow for a more realistic calculation of the behavior of sub-Neptunes.

“It is amazing how, as in this case, physical properties at molecular levels influence large-scale astronomical processes such as the formation of planetary atmospheres,” MPIA team member and Director Thomas Henning said in the statement.

Distribution of sizes of exoplanets showing a gap in planetesimals between planets 1.6 times and 2.2 times that of earth

Distribution of sizes of exoplanets showing a gap in planetesimals between planets 1.6 times and 2.2 times that of earth

The size distribution of exoplanets shows a gap in the planets between 1.6 times and 2.2 times that of Earth. (Image credit: R. Burn, C. Mordasini / MPIA)

The team found that moving toward a parent star had very different effects on a super-Earth and a mini-Neptune.

Mini-Neptunes are born away from their parent stars in the icy outer regions of their systems. While some of these planets remain in this birthplace, receiving low doses of radiation from their star, their icy material would have been melted by a mini-Neptune that migrates towards its star, the team realized.

This would create a thick atmosphere of water around these exoplanets, which would increase their radii and shift their width over the planets in the radius gap. This is possible because current observations of life outside the solar system cannot distinguish between an exoplanet’s atmosphere and solid parts when calculating width. Scientists therefore estimate that this effect is the reason for the peak of an exoplanet 2.4 times larger than Earth.

On the other hand, Super Earths that migrate towards their host star or are born very close have their atmosphere destroyed by the intense radiation of their star. That would force the worlds to lose their atmosphere and become smaller. This effect, the team says, would create a bare rocky core and a peak in exoplanet sizes 1.4 times that of Earth.

Simply put, that means that while mini-Neptune is moving out of the radial valley one way, a Super-Earth is being evacuated through the other exit. And the result of both mechanisms is the scarcity of planets about twice the width of Earth.


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— 2 ‘Super Earth’ exoplanets spotted in habitable zone of nearby star

The team’s simulations used to solve this mystery could have an impact on other areas of exoplanet science.

“If we were to extend our results to colder regions, where there is liquid water, this could suggest the existence of water life with deep oceans,” Christoph Mordasin, team member and head of the Division for Space Research and Planetary Sciences at the National University of Ireland. Bern, said in the statement. “Such planets could host life and would be relatively simple targets for searching for biomarkers thanks to their size.”

The team’s research was published on February 9 in the journal Nature Astronomy.

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