Scientists have found some planets may migrate toward the hearts of their planetary systems early on in their lives, possibly explaining the lack of planets we see that are around twice the width of Earth.
Over the years, scientists have managed to observe many exoplanets that are either smaller or larger than Earth, but planets precisely between 1.6 and 2.2 times the size of our world are relatively scarce. In particular, exoplanets defined as super-Earths or mini-Neptunes appear to be missing in space. They’re classified as such if they’re slightly larger than twice the size of our planet, but still smaller than the ice giant Neptune.
The absence of these planets has thus become known as the “radius valley” or “radius gap,” and has troubled scientists for a long time.
But now, new research suggests the “missing” super-Earths and mini-Neptunes may have just taken different routes out of the radius 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 exoplanet expert at the Max Planck Institute for Astronomy, said in a statement.
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Scientists have known for many years that planets could either move toward or away from their parent stars after formation, but what wasn’t previously known was just how effective this migration would be in creating the radius valley.
The most common explanation for the valley, Burn says, has to do with stars irradiating the planets that closely surround them, stripping the atmospheres of those worlds and causing them to shrink. This theory on its own, however, wasn’t satisfactory for him. “This explanation neglects the influence of planetary migration,” he explained.
Thus, Burn led a team of researchers that set out to investigate whether planetary migration could supplement the standard explanation and further explain the reason so few super-Earths and mini-Neptunes are seen orbiting close to 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 can’t quite study either up close. Howeer, researchers are fairly sure 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…
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