(ORDO NEWS) — Kepler-854b, Kepler-840b and Kepler-699b appear too big to be exoplanets, according to a new analysis using revised characteristics. This means they must be stars. The fourth object, Kepler-747b, is a borderline case that may require more information to resolve.
This find takes us only slightly away from the milestone of 5,000 confirmed exoplanets, which is in beckoning reach at the time of writing; but it also means we can be more confident in our exoplanet diagnoses in the future.
“Overall, this study makes the current list of planets more complete,” says astrophysicist Avi Sporer of MIT’s Kavli Institute for Astrophysics and Space Studies.
“People rely on this list to study the population of planets as a whole. If you use a sample that has multiple interpolators, your results may be inaccurate. So it’s important that the list of planets isn’t polluted.”
The boundaries between the masses of planets and stars can be a little blurry, with some overlap between them, but there are limits. Below a certain limit, objects become too small to create the core pressure and temperature needed to fuse the hydrogen that powers the star. Above a certain limit, the object must be a star.
“Most exoplanets are Jupiter-sized or much smaller,” explains astronomer Prajwal Niraula of the Massachusetts Institute of Technology, who led the study. “Twice the [size] of Jupiter is already suspect. A larger size cannot be a planet.”
The planet-finding telescope Kepler, which closed its eyes to the stars in October 2018, has been looking for exoplanets using transits. This is when an exoplanet passes between us and the host star, causing regular faint dips in starlight. This creates a “transit curve” in the star’s light that allows scientists to infer the exoplanet’s size.
With the improvement of instruments and methods, scientists began to use the so-called phase curve to study exoplanets. It includes the light from the star that the exoplanet reflects as it orbits, giving more information about the orbiting body.
Initially, Nyraula and his team studied phase curves looking for exoplanets that had been stretched into the shape of a football as a result of gravitational interaction with the host star.
This deformation can provide clues about how massive objects are and can be used to determine whether a two-body system consists of a star and an exoplanet, or a star and a smaller star.
Kepler-854b was the first hint that something might be wrong.
“Suddenly we had a system where we saw a huge ellipsoidal signal, and we knew right away that it couldn’t be from a planet,” Sporer says. Then we thought, “Something doesn’t add up here.”
The depth of the transit curve signal is related to the size ratio of the exoplanet and host star. If you know the size of a star – which isn’t always easy to figure out because it’s related to distances, and they’re hard to determine in space – you can infer the size of an exoplanet.
A project called Gaia is changing our understanding of the Milky Way. Using stellar parallax, Gaia displays the exact position and movement of the stars in the Milky Way in 3D space with the highest precision. In 2016, when Kepler-854b was discovered, Gaia data for its host star were not available.
However, now they are; when Nyraula and colleagues revised the properties of the exoplanet using updated Gaia data, they found that the exoplanet is much larger than previously thought, about 3 times the size of Jupiter. They also calculated its mass – about 239 times the mass of Jupiter; the upper limit for a planet’s mass is about 10 Jupiters.
“There’s no way the universe could create a planet this size,” Sporer said. “It just doesn’t exist.”
Knowing that tiny stars could be lurking in Kepler’s database of roughly 2,000 exoplanets, the researchers set out to find more.
Kepler-840b was 2.5 times the size of Jupiter, and Kepler-699b was 2.76 times the size of Jupiter. Kepler-747b was a borderline case – 1.84 times the size of Jupiter. (Remember that more than twice the size of Jupiter is suspicious.)
Now that the problem has been identified, it’s unlikely that there are many more tiny stars masquerading as confirmed exoplanets, the team says. With a wealth of Gaia data at our disposal and an awareness of the problem, we can be more confident that exoplanets are exoplanets.
“It’s a tiny correction,” Sporer says. “It comes from a better understanding of the stars, which is improving all the time. So the likelihood that the radius of the star will turn out to be so wrong is much less. Such classification errors will not be repeated many times.”
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