Researchers have found that certain planets outside the solar system are shrinking due to the loss of their atmosphere
Certain planets outside the solar system appear to be losing their atmospheres and shrinking. In a new study using the Kepler space telescope, astronomers find evidence for a possible reason: the cores of these planets are pushing their atmospheres from the inside out.
Extrasolar planets range in size from small rocky planets to huge gas giants. In between are rocky super-Earth planets and larger sub-Neptunian planets with bloated atmospheres. But there is a suspicious absence - a "size gap" - of planets that are 1.5-2 times the size of Earth (or between Super-Earth and Sub-Neptune) that scientists are trying to understand better.
"Scientists have so far confirmed the discovery of more than 5,000 exoplanets, but there are fewer planets than expected that are 1.5-2 times the diameter of Earth," said Caltech/IPAC research scientist Jessie Christiansen, scientific director of the NASA Planetary Archive. A and lead author of the new study in The Astronomical Journal. "Exoplanet scientists now have enough information to say that this gap is no accident. Something happens that prevents planets from reaching this size or staying at it."
The scientists think that a possible explanation for this discrepancy is that some sub-Neptunian planets lose their atmosphere over time. This loss will happen if the planet does not have enough mass, and therefore gravity, to maintain its atmosphere. Sub-Neptunians that are not massive enough will therefore shrink to roughly the size of a super-Earth, leaving a gap between the two planet sizes.
But how exactly these planets lose their atmosphere is still a mystery. The scientists converged on two possible mechanisms: one called core-driven mass loss and the other photoevaporation. The study revealed new evidence supporting the first mechanism.
This video explains the differences between the main types of exoplanets. Credit: NASA/JPL-Caltech
Core-driven mass loss occurs when radiation emitted from a planet's hot core pushes the atmosphere away from it over time, "and that radiation pushes the atmosphere from below," Christiansen said.
The second leading explanation for the planetary gap, photoevaporation, occurs when a planet's atmosphere is actually blown out by the hot radiation of its host star. In this scenario, "the high-energy radiation from the star acts like a hair dryer on an ice cube."
It is believed that photoevaporation occurs in the first hundred million years of a planet, while core-driven mass loss occurs much later - closer to a billion years after its formation. But in both mechanisms, "if there isn't enough mass, you can't hold on and you lose the atmosphere and shrink," Christiansen added.
In this study, Christiansen and her colleagues used data from NASA's K2, an extended Kepler space telescope mission, to observe the Beehive and Evaporation star clusters, which are 600 to 800 million years old. Planets are generally thought to be the same age as their host star, so the sub-neptunities in this system would be beyond the age where photoevaporation could occur but below the age of core-driven mass loss.
So if they see that there are a lot of sub-neptunities in the clusters (compared to older stars in other clusters), they can conclude that photoevaporation did not occur. In this case, core-driven mass loss would be the most likely explanation for what happened to less massive sub-neptunities over time. Observations found that nearly 100% of the stars in these clusters still have sub-Neptunian planets orbiting them. Based on their size, researchers think they have retained their atmosphere.
More of the topic in Hayadan: