Positioned throughout the universe, there are billions — possibly even trillions — of exoplanets orbiting around stars of various shapes, sizes, colours, and more. Like the celebs they orbit, exoplanets also are available many alternative shapes, sizes, and colours, with scientists classifying exoplanets into considered one of 4 groups: gas giants, super-Earths, sub-Neptunes, and terrestrial.
Interestingly, among the many 5,000+ exoplanets which have been discovered and cataloged by NASA, ESA, and other agencies, there may be a wierd absence of exoplanets whose sizes are between 1.5 and two times the scale of Earth (between super-Earths and sub-Neptunes). In a brand new study using data from NASA’s now-retired Kepler Space Telescope, scientists could have found evidence for why this size gap exists — the cores of the exoplanets are pushing away their atmospheres from the within out.
“Scientists have now confirmed the detection of over 5,000 exoplanets, but there are fewer planets than expected, with a diameter between 1.5 and two times that of Earth. Exoplanet scientists have enough data now to say that this gap is just not a fluke. There’s something occurring that impedes planets from reaching and/or staying at this size,” said the science lead for the NASA Exoplanet Archive and lead creator Jessie Christiansen of Caltech.
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Graphic showing 4 several types of exoplanets. (Credit: NASA/JPL-Caltech)
As mentioned, the gap in exoplanet size lies between the sizes of super-Earths and sub-Neptunes. Scientists consider that the sub-Neptune exoplanets are the possible explanation for the scale gap, as previous studies have shown that sub-Neptunes are liable to atmospheric loss. The exoplanets can lose their atmospheres in the event that they don’t have enough mass, and due to this fact enough gravitational force, to maintain their atmospheres. If this theory of atmospheric loss is true and the sub-Neptunes don’t have enough mass to carry on to their atmospheres, they’d likely shrink to the scale of super-Earths, explaining the scale gap between super-Earths and sub-Neptunes.
Nonetheless, the precise process by which sub-Neptunes lose their atmospheres has remained a mystery for years. The 2 leading theories are core-powered mass loss and photoevaporation. The brand new Kepler study from Christiansen et al. has shown evidence for the primary theory: core-powered mass loss.
As mentioned, core-powered mass loss is the method by which a planet’s core pushes away the planet’s atmosphere from the within out. The phenomenon occurs when radiation emitted by the planet’s hot core interacts with the planet’s atmosphere, causing the atmosphere to fade away slowly.
The opposite theory behind atmospheric loss in sub-Neptunes, photoevaporation, occurs when the radiation from the exoplanet’s host star, akin to solar wind and flares, blows away the atmosphere across the exoplanet. Dr. Christiansen says, “the high-energy radiation from the star is acting like a hair dryer on an ice cube.”
Scientists consider photoevaporation occurs throughout the first 100 million years of an exoplanet’s life, while core-powered mass loss is believed to occur much later — around one billion years after the formation of the planet. Either way, if the planet doesn’t have enough mass, it can lose its atmosphere and shrink.
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Artist’s impression of a sub-Neptune exoplanet losing its atmosphere via photoevaporation. (Credit: W. M. Keck Observatory/Adam Makarenko)
Throughout the study, Christiansen et al. used data from NASA’s K2 mission, an prolonged mission of the Kepler Space Telescope, which was retired in 2018. The team used data Kepler collected on star clusters Praesepe and Hyades, that are 600 million to 800 million years old, respectively.
Given those exoplanets are regarded as around the identical age as their host star, Christiansen et al. knew that in the event that they observed exoplanets throughout the two-star clusters, the planets ought to be sufficiently old to have experienced photoevaporation but still too young to have experienced core-powered mass loss. The team predicted that in the event that they saw a high variety of sub-Neptunes within the star clusters, they might conclude that photoevaporation had not occurred — meaning that core-powered mass loss could be the leading explanation behind atmospheric loss in sub-Neptunes.
So, what did the team find within the K2 data?
Christiansen et al. found that almost all of the celebs in Praesepe and Hyades still have sub-Neptunes or other exoplanets with atmospheres in orbit around them. After investigating the sizes of the exoplanets around the celebs, the team believes that lots of the exoplanets still have their atmospheres.
The existence of those exoplanets around these stars differs from the older stars K2 observed, which were older than 800 million years old. Of those older stars, it was found that only 25% have sub-Neptunes of their orbits. Interestingly, the older ages of those stars are closer to the timeframe wherein it is believed that core-powered mass loss occurs.
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Image of the Praesepe star cluster. (Credit: Stuart Heggie)
Christiansen et al.’s results allowed the team to conclude that photoevaporation couldn’t have occurred inside Praesepe and Hyades, as there could be only a few exoplanets with atmospheres throughout the star clusters if photoevaporation had occurred. Because of this core-powered mass loss is the leading theory behind atmospheric loss in sub-Neptunes.
It took Christiansen et al. greater than five years to create the catalog of exoplanets that was utilized in this study. While the team’s findings are definitely telling, there remains to be lots to find out about photoevaporation and core-powered mass loss. Moreover, upcoming studies into sub-Neptunes and atmospheric loss in exoplanets will put Christiansen et al.’s findings to the test.