Like the planets in our solar system, exoplanets come in various sizes and types. But some apparently don\u2019t stay<\/em> the same size. In fact, some exoplanets appear to be able to shrink<\/em>. How is that possible? Scientists at NASA, Caltech and other institutions said on November 15, 2023, that they might have an answer. They said some sub-Neptune exoplanets \u2013 gaseous worlds with smaller radii than our solar system\u2019s planet Neptune, but larger than Neptune in mass and so more dense \u2013 have cores that are pushing away their atmospheres from the inside out<\/em>. So these gaseous planets are losing their atmospheres. And thus they\u2019re shrinking<\/em> in size. <\/p>\n The existence of shrinking sub-Neptunes might explain the unusual size gap between them and super-Earths, which are more massive than Earth, but less massive than Neptune.<\/p>\n That is, some sub-Neptunes simply become<\/em> super-Earths.<\/p>\n The researchers published their peer-reviewed findings in The Astronomical Journal<\/em> on November 15, 2023.<\/p>\n After discovering more and more exoplanets over the past few decades, astronomers have noticed something odd. Both rocky super-Earths and larger gaseous sub-Neptunes are fairly common. But there is a distinct lack of exoplanets between those two sizes. This size gap \u2013 of planets 1.5 to two times the size of Earth \u2013 has puzzled astronomers. What is causing it?<\/p>\n The 2024 lunar calendars are here! Best Christmas gifts in the universe! Check \u2019em out here.<\/p>\n As lead author Jessie Christiansen at Caltech in Pasadena, California, explained:<\/p>\n 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 not a fluke. There\u2019s something going on that impedes planets from reaching and\/or staying at this size.<\/p>\n<\/blockquote>\n The new findings suggest that shrinking exoplanets can explain the mysterious size gap. Some larger sub-Neptune planets shrink to become super-Earths, leaving no intermediate sized planets. Hence the gap in sizes. But how is that happening?<\/p>\n The researchers said that if a sub-Neptune doesn\u2019t have enough mass, it could start to lose some of its atmosphere. This is due to not enough gravitational force to hold onto all of the atmosphere. By losing some or much of their outer atmospheres, these planets would \u201cshrink\u201d in overall size.<\/p>\n That can explain the size differences, but exactly how<\/em> are these planets\u2019 atmospheres being decimated? There have been two main theories posited to date: core-powered mass loss and photoevaporation.<\/p>\n In core-powered mass loss, the planet\u2019s hot core releases radiation. That radiation \u201cpushes out\u201d the atmosphere from underneath over time. Photoevaporation is kind of the opposite. Radiation from the planet\u2019s host star blows away the outer atmosphere. Christiansen said that:<\/p>\n \u2026 the high-energy radiation from the star is acting like a hair dryer on an ice cube.<\/p>\n<\/blockquote>\n There\u2019s another difference between the two scenarios. Scientists think that photoevaporation occurs early in the planet\u2019s history, within the first 100 million years. Core-powered mass loss, however, doesn\u2019t happen until about a billion years later. Both can lead to a loss of atmosphere. Christiansen said:<\/p>\n If you don\u2019t have enough mass, you can\u2019t hold on, and you lose your atmosphere and shrink down.<\/p>\n<\/blockquote>\n Either process could explain a lost atmosphere. So, how did the researchers determine which of the two possibilities is more likely? They used data from the Kepler Space Telescope\u2018s extended mission, called K2. They focused on two star clusters specifically, Praesepe (also known as the Beehive cluster or M44) and the Hyades. Both are known to contain many sub-Neptunes.<\/p>\n The ages of the stars in these clusters is of particular importance. The stars are between 600 million and 800 million years old. While still relatively young, their sub-Neptunes \u2013 about the same age \u2013 are old enough that photoevaporation would have already occurred several hundred million years ago. But they are not<\/em> old enough yet for core-powered mass loss.<\/p>\n With this in mind, the researchers reasoned that if there were still many full-sized sub-Neptunes in these clusters, then they couldn\u2019t have experienced photo evaporation. And indeed, they found that nearly all the stars in the clusters have a sub-Neptune or at least a candidate. They are still full-sized, meaning they have held onto their atmospheres.<\/p>\n Meanwhile, in some older star clusters, only about 25% of those stars have sub-Neptunes. This fits with the scenario that core-powered mass loss would have occurred by that point. That would result in fewer sub-Neptunes and more super-Earths. If photo evaporation had happened in Praesepe and the Hyades, then those planets should have lost much of their atmospheres. But they didn\u2019t.<\/p>\n The results leave core-powered mass loss as the most likely explanation for the shrinking planets. But, as the press release said, the research is far from complete, so:<\/p>\n \u2026 it is possible that the current understanding of photoevaporation and\/or core-powered mass loss could evolve. The findings will likely be put to the test by future studies before anyone can declare the mystery of this planetary gap solved once and for all.<\/p>\n<\/blockquote>\n Bottom line: Scientists say they may have solved the mystery of shrinking exoplanets. Radiation from the cores of less massive sub-Neptunes pushes away their atmospheres.<\/p>\n Read more about the differences between super-Earths, mini-Neptunes and sub-Neptunes.<\/p>\n Source: Scaling K2. VII. Evidence For a High Occurrence Rate of Hot Sub-Neptunes at Intermediate Ages<\/p>\n Via NASA<\/p>\n Read more: How shrinking planets might explain missing planets<\/p>\n Read more: Super-Earth and mini-Neptune in synchronized dance<\/p>\n <\/div>\nA size gap in exoplanets<\/h3>\n
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Shrinking exoplanets<\/h3>\n
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Star clusters provide clues to shrinking exoplanets<\/h3>\n
Core-powered mass loss most likely explanation<\/h3>\n
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