A new study led by researchers at the Universities of Chicago, Michigan and Maryland suggests that helium can accumulate in the atmosphere of exoplanets. If confirmed, the discovery could explain a decades-old enigma about the size of these worlds, also contributing to the understanding of planetary evolution.
New telescopes and methods developed in recent decades have led to the discovery of thousands of exoplanets (worlds that orbit other stars), with very different composition and sizes. Today, astronomers estimate that at least half of all Sun-like stars are orbited by at least one exoplanet.
These worlds are between the size of Earth and Neptune, with atmospheres of hydrogen and helium. However, when analyzing the numbers of exoplanets, scientists noticed something interesting: they can be divided into groups with one exoplanet 1.5 times the size of Earth and another that contains worlds twice the diameter of ours or more.
There are almost no “intermediate” sized exoplanets between these two groups, which forms a gap called the “lightning valley”. For the authors of the new study, the valley may be related to the atmospheres of the planets – that’s what explains Isaac Malsky, lead author. “For example, perhaps the smallest group of planets have lost their atmospheres entirely, and exist only as rocky cores,” he suggested.
To analyze how heat and radiation from the star might affect exoplanet atmospheres, they developed models based on exoplanet data and laws of physics. The team simulated around 70,000 exoplanets with varying sizes and atmospheres with varying temperatures, orbiting diverse stars, and modeled what would happen to them over time.
They found that, after a few billion years, hydrogen from the planetary atmosphere can escape more quickly than helium, resulting in the accumulation of the latter. If the theory is correct, it means that planets with helium-rich atmospheres should be common in the larger exoplanet pool, as a consequence of the gas accumulating as they shed their atmospheres.
In addition, groups of exoplanets of different sizes can be formed due to small amounts of hydrogen and helium, sufficient to form atmospheres capable of “swelling” the radius of the exoplanet. If they retain any atmosphere, they can be classified in the group of major planets; otherwise, they remain in the group of minor worlds. The results can be confirmed with observations from space telescopes such as the James Webb.
The article with the results of the study was published in the journal Nature Astronomy🇧🇷