The mystery of the missing super-Earths and mini-Neptunes may finally be solved

Scientists have discovered that some planets migrate to the hearts of their planetary systems early in their lives, which may explain the lack of planets we see that are about twice the width of Earth.

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

The absence of these planets has therefore become known as the ‘radius valley’ or ‘radius gap’, and has long concerned scientists.

But now new research suggests that the ‘missing’ super-Earths and mini-Neptunes may have just taken different routes out of the jet valley.

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

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Scientists have known for years that planets can move either toward or away from their host star after formation, but what was previously unknown was how effective this migration would be in creating the jet valley.

The most common explanation for the valley, according to Burn, has to do with stars irradiating the planets that surround them, stripping those worlds of their atmosphere and causing them to shrink. However, this theory in itself was not satisfactory to him. “This explanation ignores the influence of planetary migration,” he explained.

For example, Burn led a team of researchers who wanted to investigate whether planetary migration could supplement the standard explanation and further explain why so few super-Earths and mini-Neptunes orbit near their stars.

Super-Earths shrink while sub-Neptunes grow

Because the two planets that occupy the radius gap, super-Earths and mini-Neptunes, are both absent from the solar system, scientists cannot study them up close either. However, researchers are quite certain that super-Earths are rocky or terrestrial planets, while the characteristics of mini-Neptunes are much less certain.

What scientists also agree on is that mini-Neptunes, also known as sub-Neptunes, should have atmospheres far beyond those of rocky planets.

Burn and the team wanted to know whether this fact could play a role in the creation of the jet valley, and whether the mere existence of that valley could suggest very different formations and evolutions for super-Earths and mini-Neptunes.

In a reanalysis of a simulation the team ran in 2020, Burn and colleagues took into account processes in the gas and dust disks around young stars that give rise to new planets, the formation of atmospheres and the migration of planets.

Critical to the simulation and to developing a possible solution for the radius valley – which also plays a role in planetary migration – was understanding how water behaves over a wide range of pressures and temperatures. That’s because these parameters allow a more realistic calculation of the behavior of sub-Neptunes.

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

A distribution of exoplanet sizes showing an inter-planet gap between planets that is 1.6 times to 2.2 times the size of Earth

A distribution of exoplanet sizes showing an inter-planet gap between planets that is 1.6 times to 2.2 times the size of Earth

A distribution of exoplanet sizes showing a gap in planets between 1.6 times and 2.2 times the size of Earth. (Image credit: R. Burn, C. Mordasini / MPIA)

The team found that motion toward a parent star had drastically different effects on both super-Earths and mini-Neptunes.

Mini-Neptunes are born away from their parent stars in the icy outer reaches of their system. While some of these planets remain at this birthplace and receive low doses of radiation from their star, the icy material of mini-Neptunes that do migrate to their star should be thawed, the team realized.

This would create a thick water atmosphere around these exoplanets, increasing their radius and shifting their width beyond the planets in the jet gap. This is possible because current observations of worlds outside the Solar System cannot distinguish between an exoplanet’s atmosphere and solid parts when calculating latitude. This effect, the scientists estimate, will cause a spike in exoplanets 2.4 times the size of Earth.

On the other hand, super-Earths that migrate to their parent stars or are born very close by would have their atmospheres stripped by the intense radiation from their stars. That would force the worlds to lose their atmosphere and become smaller. This effect, the team says, would create a bare rocky core and cause a spike in exoplanet sizes of just 1.4 times the width of Earth.

Simply put, this means that while mini-Neptunes exit the jet valley in one direction, super-terrestrials evacuate it through the opposite exit. And both mechanisms result in a shortage of planets about twice as wide as Earth.


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– 2 ‘super-Earth’ exoplanets spotted in the habitable zone of a nearby star

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

“If we were to extend our results to cooler regions, where water is liquid, this could indicate the existence of water worlds with deep oceans,” says Christoph Mordasin, team member and head of the Department of Space Research and Planetary Sciences at the University of New York. Bern, said in the statement. “Such planets could potentially harbor life and, thanks to their size, would be relatively easy targets for the search for biomarkers.”

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

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