Shrinking instead of missing? Astronomers may have discovered why there is a mysterious size gap between super-Earths and mini-Neptunes on exoplanets. Young planets in two nearby star clusters provided crucial hints: More than 80 percent of these young exoplanets belong to the missing intermediate type with one to two Earth sizes, as the team discovered. This suggests that this planetary gap emerges subsequently and also provides initial insights into the cause.
In the size spectrum of planets, there is a conspicuous gap: there are hardly any exoplanets in the size range between super-Earths with up to 1.75 times the Earth’s radius and gas-rich mini-Neptunes with two to four times the Earth’s radius. “Exoplanet scientists have enough data now to say that this gap is not a fluke,” explains lead author Jessie Christiansen from NASA’s Exoplanet Science Institute. “There’s something going on that impedes planets from reaching and/or staying at this size.”
But what is it? Astronomers now suspect that the missing intermediate type does emerge but is not stable. Because these exoplanets have relatively low mass and gravity, they can hold onto their dense primordial atmospheres less effectively than the heavier mini-Neptunes.
Two Possible Scenarios
However, this raises the question of how the gas envelopes of such young planets are lost. So far, there are two possible explanations: In the first scenario, the high-energy radiation from the young parent stars strips away the atmosphere of these gas-rich young planets. Astronomers discovered evidence of this in 2022 for two young sub-Neptunes. However, according to prevailing models, such photoevaporation should occur within the first approximately 100 million years after planet formation.
In the second scenario, the young exoplanets do not lose their gas envelope due to their star but rather through their own heat: their core, which can be as hot as 100,000 Kelvin, emits an enormous amount of heat radiation that impacts the planetary gas envelope from the inside. This gradually thins the envelope, causing it to lose gas to the surrounding space. Unlike photoevaporation, this core-induced mass loss can persist for several billion years. Observations indeed show that the relative size of sub-Neptunes decreases with their age.
A Look into Two Young Star Clusters
However, which of these two processes is responsible for the size gap between super-Earths and mini-Neptunes? Or could it possibly be both? This is where the study by Christansen and her team comes in. Their idea is to test whether the size gap should be measurable on planets more than 100 million years old if photoevaporation is the main cause. On the other hand, if the gas loss is core-induced, the gap should still be absent on slightly older planets.
To test this, astronomers examined exoplanets in the two nearby young star clusters, Praesepe and Hyaden. Both contain several hundred young stars that are only 600 to 800 million years old. The Kepler Space Telescope detected 15 young exoplanets in these star clusters. Christansen and her team then determined whether these planets fall into the size gap and what the proportion of such planetary intermediate forms is.
Abundance of Planetary Intermediate Sizes
The surprising result: Almost all young exoplanets in these two star clusters are hot sub-Neptunes of the missing intermediate size; they constitute more than 80 percent of the exoplanets there. “This is significantly more than in the entire star catalog observed by Kepler,” report the astronomers. In older stars, three to nine billion years old, only about 17 percent of exoplanets fall in the size range between 1.2 and 3.9 Earth radii.
The young planets in the Hyaden and Praesepe star clusters orbit their parent stars very closely but do not seem to have lost their gas envelope, even though they are already 600 to 800 million years old. According to Christansen and her colleagues, this argues against the photoevaporation scenario because, according to it, the gas loss of these young planets should already be completed.
Indication of Mass Loss Due to Core Heat
Instead, the astronomers see a strong indication of the slower core-induced atmospheric loss in their observations. “Our result for the Praesepe and Hyaden star clusters is more in line with theoretical models for mass loss due to core heat,” they conclude. According to this, the planetary size gap does not occur directly after planet formation but gradually as the young planets cool down and radiate their heat outward.
However, these observations alone are not sufficient to completely rule out other scenarios, as the team emphasizes. There could be factors, for example, that slow down photoevaporation. A combination of both processes or even a completely different explanation, is also conceivable. Further investigations are therefore necessary, according to the astronomers.