This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A new study suggests that some of the heaviest rocky planets ever reported may not be as heavy as they look. These so-called 'mega-Earths,' rocky planets weighing more than about 10 Earths, could be an illusion created by activity on their host stars.
The paper was posted to the arXiv preprint server on Oct. 8. Planets stop being bare rocky worlds and start developing a gas envelope by accreting gas from their disk at roughly 1.5 to 1.75 times Earth's radius, corresponding to about 4 to 5 Earth masses. Theory suggests a much higher upper limit on this mass range.
Even then, simulations suggest a purely rocky planet is unlikely to grow beyond about 10 Earth masses. This is because a core that massive should be pulling a thick envelope of gas from the disk it formed in. Yet astronomers have found a few planets above this threshold that are dense and gas-free.
If they are real, they would need exceptional pathways of formation, such as collisions after the gas disk disappeared, or they would have to be the stripped cores of former giant planets. Astronomers also look at another possibility: the host star's activity. Planet masses are measured using the radial-velocity (RV) method, which measures how much a planet tugs its star back and forth.
Stars with strong magnetic activity make it hard to measure the mass accurately this way. As star spots and bright patches on their surfaces rotate in and out of view, they produce wobbles that can imitate a planet's gravitational pull. This stellar 'noise' can drown out the faint signal of a small planet or sometimes exaggerate it, so planets may end up with mass estimates that are too high.
In this study, astronomers Casey Brinkman and Nicolas Cowan set out to test whether the mega-Earths are real. They started with small planets under four times Earth's size from NASA's Exoplanet Archive and kept only those whose masses were measured by the radial-velocity method. Dropping planets with weak or indirect mass estimates, they were left with 333 planets in their sample.
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