Astronomers studying a planet 245 light-years from Earth believe they may have found the missing piece in a long-standing puzzle surrounding how other worlds are formed.
Typically, exoplanets fall into one of two categories based on their size. “Super-Earths” are typically no more than around 1.5 times the size of our planet. So-called “mini-Neptunes, meanwhile, have a radius between two–four times that of the Earth.
In between these two lies what experts have dubbed “radius valley”, with planets lying between 1.5 and two times the size of the Earth appearing to be in short supply.
Using data from NASA’s Transiting Exoplanets Survey Satellite (TESS), however, an international team of researchers has detected a new planet — TOI-733b — in this gap.
TESS uses the “transit method”, detecting the presence of distant planets by how the light from their parent star dims as the planet passes between the star and us.
TOI-733b has a radius almost twice that of Earths, and lies close to its star, taking just 4.9 days to complete a single orbit of its parent.
The study of the newly-identified world was undertaken by astronomer Dr Iskra Georgieva, of the Chalmers University of Technology in Gothenburg, Sweden, and her colleagues.
Georgieva said: “TOI-733b is the most interesting exoplanet I have studied, since it is a planet in the radius valley, orbiting a Sun-like star. That is rare!
“There are several different theories on how these planets form and develop.”
Based on their measurements and models of TOI-733b, the researchers believe that the world likely originally had an atmosphere of light elements like hydrogen and helium — but such has since been lost.
Planets can lose their atmospheres in several ways. The most common mechanisms, however, are “photo-evaporation” and “core-powered mass loss”.
The former, and by far the faster, involves radiation from a nearby star burning off the atmosphere early in the star system’s life, while the latter involves evaporation driven by the release of geothermal heat from the planet’s core.
Georgieva added: “We think that planets like TOI-733b represent a critical size in planet formation.
“Planets that reach this size quickly attract thick atmospheres of hydrogen and helium gas, and balloon up into gaseous planets — like Neptune.
“Whereas planets smaller than this limit can’t hold on to such an atmosphere and therefore remain primarily rocky, with mostly thinner envelopes.
“And if they are close to their star, which in turn emits the right amount and kind of radiation, the planets’ atmospheres could be stripped away by the star. This appears to be the case for TOI-733b.”
Exactly how planets lose their atmospheres is a key conundrum in our understanding of planetary evolution — one that TOI-733b could help shine a light on.
Georgieva added: “Do these planets lose their atmospheres in one of the above-described ways? Can both processes occur, or is one more dominant? How often do planets of this size end up like TOI-733b? There are a lot of questions left to answer!”
The full findings of the study were published in the journal Astronomy & Astrophysics.
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