Mini-Neptune exoplanets could shed their atmospheres as they age, thus ‘leaping the radius gap’ and explaining why mid-sized planets seem to be missing from our catalogs.
Some of the Universe’s planets are missing! This strange statement reflects that while astronomers and extra-solar planet (exoplanet) hunters have had great success discovering rocky super-Earths and gas giant mini Neptunes outside the solar system, planets with sizes that fall between these two types of worlds seem to be very uncommon.
Amongst the thousands of exoplanets discovered thus far, there exists a significant ‘radius gap’ in our exoplanet catalog, with exoplanets possessing a radius of between 1.5 and 2 that of Earth ‘missing.’
There’s no significant reason why the Universe should lack planets of such a size, so this gap — officially titled the small planet radius gap or the Fulton gap — has troubled astrophysicists since 2017. New research could have found a reason for this disparity.
Scientists led by Trevor David of the Center for Computational Astrophysics at the Flatiron Institute, New York, have reassessed astronomical data classifying exoplanets by age.
In doing this, the team discovered that planets’ radii seem to change with age, with older mini-Neptune planets appearing to have shrunk radically over their lifetimes, leaving behind just a solid core. Thus these older planets shed gas ‘jump the radius gap’, leaving behind super-Earths and explaining why midsize planets are so rare.
“The overarching point is that planets are not the static spheres of rocks and gas we sometimes tend to think of them as,” says David, one of the authors of a paper published in the latest edition of the Astronomical Journal¹. David explains in prior research, “Some of these planets were 10 times larger at the starts of their lives.”
Leaping the Radius Gap
The fact that the radii of discovered exoplanets had a strongly bimodal distribution was noted as early as 2012, but it was first precisely defined by Benjamin J Fulton, now a Research Scientist at the NASA Exoplanet Science Institute, in his doctoral thesis.
It isn’t so much that exoplanets with a radius between 1.5 and 2 Earth radii don’t exist, or we haven’t found any of them. It’s just that we haven’t found anywhere near enough if they are as common as their larger and smaller counterparts.
David and his team divided exoplanets into ‘young’ and ‘old’ sub-categories — with young planets being defined by the researchers as worlds younger than 2 billion years old. These ages were defined by assessing the age of the planet’s parent star as planets and their stars form within a relatively short time — cosmically speaking.
Thus, the team found a distinct lack of young planets with 1.6 times the radii of earth, whilst the least common radii amongst the older planets were 1.8 that of our planet.
This implies that their lifetimes of billions of years mini-Neptunes lose their atmospheres by leaking gas away to space. This happens until all that is left is a small solid core. As time has progressed in the Universe, larger and larger mini-Neptunes have made this radius jump. This results in the creation of progressively larger super-Earths.
The Fulton gap — that valley that exists in the bimodal distribution of exoplanet radii — is actually the radius difference between the largest super-Earths and the smallest mini-Neptunes, which still possess their gaseous atmospheres.
The team took measurements of light from distant stars collected by NASA’s Kepler Space Telescope to conduct their study — now deactivated. These light profiles show how light from a star dims by a tiny amount when an exoplanet traverses its face. This dimming reveals the size of the exoplanet, analysis that led to the initial discovery of the radius gap.
By cleaning up this data the team also found that the valley in exoplanet radii is more pronounced than previously believed. Their results could help to distinguish between previous suspects in this ‘missing planet mystery.’
The Key Suspects in the Missing Planet Mystery
Astronomers had previously pointed the finger at mechanisms that can cause gases to be stripped away from planets as the reason for the Fulton gap. The major difference between these mechanisms is the timescale over which they operate.
Astrophysicists had previously suggested that some planets could form without enough gas in close proximity to provide a ‘puffed-up’ atmosphere and thus, radius. That would mean the radius gap is ‘built-in’ at the birth of these worlds.
Other theories have pointed towards the impact of asteroids, comets, and other space debris blasting away atmospheres and even preventing gas from accumulating in the case of smaller planets. This would take anywhere between 10 to 100 million years.
Other processes like photoevaporation, the stripping of a planet’s atmosphere by its host star’s intense emission of radiation, could take a similar amount of time for smaller planets — around 100 million years — but much longer, around a billion years, for larger worlds. The gradual addition of heat from the planet to its atmosphere would be a similarly long-term process.
The fact that the team’s results point to a very gradual loss of radius, would seem to support these two more long-term mechanisms. This could rule out collisions with space rocks and quirks of planetary formation as causes of the Fulton gap.
“Probably both effects are important,” says David, “but we’ll need more sophisticated models to tell how much each of them contributes and when in the planet’s life cycle.”
The researchers will now attempt to discover which of these long-term mechanisms plays a more significant role in gas loss and, in turn, in the lack of mid-sized exoplanets.
1. David. T. J., Contardo. G., Sandoval. A., et al, , ‘Evolution of the Exoplanet Size Distribution: Forming Large Super-Earths Over Billions of Years,’ the Astronomical Journal, [https://iopscience.iop.org/article/10.3847/1538-3881/abf439]