Bigger rocky planets may be uncommon as a result of they’ve shrunk
Researchers at the Flatiron Institute’s Center for Computational Astrophysics published an article last week that may explain a mysterious gap in planet size beyond our solar system. Planets between 1.5 and 2 times the radius of the earth are remarkably rare. This new research suggests that this could be because planets slightly larger than these, called mini-Neptunes, lose their atmosphere and shrink over time to become “super-earths” that are only slightly larger as our home planet. These changing planets only briefly have the right size radius to fill the void and are rapidly shrinking beyond that. The implication for planetary research is exciting as it confirms that planets are not static objects, but evolving and dynamic worlds.
Exoplanet research is a very young field. In 1992 no one had seen a planet beyond our solar system. Today we have discovered more than 4,700 of them, and that number is growing rapidly due to the efforts of dedicated space telescopes for planet hunting such as Kepler (now defunct) and its successor, TESS. We have suddenly gained a huge new sample size of planets to study, beyond the eight planets (sorry Pluto) orbiting our sun.
Kepler, TESS and other planet hunters have discovered brand new types of planets, such as so-called “hot Jupiter”, large gas giants that orbit very close to their star. These were among the first observed exoplanets because their size made them easy to find, and their small, rapid orbital times, allowed them to pass in front of their star more than once in a short amount of time (some hot Jupiters did this) a year that few Earth days lasts).
Artist’s impression of KELT-9b, a hot Jupiter planet orbiting near its star. Photo credit: NASA / JPL-Caltech.
As our ability to find smaller planets has grown, we have begun to see a wide variety of types of planets, the smallest of which are even smaller than Mercury. However, as the sample size increases, the strange gap between 1.5 and 2 earth radii remains. For some reason, planets just don’t like being that big.
Previous theories suggested that asteroid bombardment could tear atmospheres off planets of this size, or that some planets could form in regions without enough gas to even create a thick atmosphere, with their overall size well below the “gap”.
The research team, led by Trevor David, approached the puzzle in a new way, considering whether the size had changed over time. Planets tend to form at the same time as their star. So if you know the age of the star, you can estimate the age of the planet. This allowed the team to sort the planets into age groups.
What they found was that older planets (older than 2 billion years) had the gap centered around 1.8 Earth radii, while planets younger than 2 billion years had a size gap closer to 1.6 Earth radii.
Courtesy of the Simons Foundation.
This difference suggests that the smallest mini-Neptunes fail to cling to their atmosphere and shrink to become super-earths pretty early. The same process occurs later with slightly larger mini-Neptunes, which leads to a shift in the “gap”. As Thomas Sumner of the Simons Foundation puts it, the void is therefore best understood as “the gap between the greatest super-earths and the smallest mini-Neptunes that can still maintain their atmosphere”.
What is the cause of this atmospheric shrinkage? This is likely due to radiation from a planet’s star blowing the gas away, or to the residual heat in the planet itself. These processes affect all planets to some extent, but the largest planets have enough gravity to have the effect is nowhere near as dramatic.
Once a puzzle is solved (or at least plausibly explained) there is much more to learn about the details of the process, such as studying how magnetic fields can affect planet size and atmospheric loss.
The paper was published in the Astronomical Journal.
Trevor David et al., “Evolution of Exoplanet Size Distribution: Formation of Great Super-Earths Over Billions of Years.” The Astronomical Journal.
Thomas Sumner, “Shrinking planets could explain the mystery of the universe’s missing worlds,” Simons Foundation.
Selected image source: Aldaron (Wikimedia Commons).