A handful of planets discovered by NASA’s Kepler Space Telescope have gotten the Pluto treatment. A recent study finds that three, and potentially four planets, were misclassified and are likely small stars.
A team of Massachusetts Institute of Technology researchers used updated measurements of planet-hosting stars to double-check the size of the planets, and concluded three were simply too big to be planets. Researchers say the three objects, known as Kepler-854b, Kepler-840b, and Kepler-699b, are estimated to be between two and four times the size of Jupiter.
“Most exoplanets are Jupiter-sized or much smaller. Twice [the size of] Jupiter is already suspicious. Larger than that cannot be a planet, which is what we found,” says Prajwal Niraula, the study’s first author and a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, in a statement.
A fourth planet, called Kepler-747b, is about 1.8 times Jupiter’s size and is relatively far from its star. Due to the distance, the amount of light Kepler-747b receives is too small to sustain a planet of its size. Researchers say its planetary status is suspect but not entirely implausible.
“Overall, this study makes the current list of planets more complete,” says Avi Shporer, study author and research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. “People rely on this list to study the population of planets as a whole. If you use a sample with a few interlopers, your results may be inaccurate. So, it’s important that the list of planets is not contaminated.”
Niraula wasn’t looking to reclassify planets as his initial intention for the study was to look for systems with signs of tidal distortion.
“If you have two objects close to each other, the gravitational pull of one will cause the other to be egg-shaped, or ellipsoidal, which gives you an idea of how massive the companion is,” explains Niraula. “So you could determine whether it’s a star-star or star-planet system, just based on that tidal pull.”
While looking through the Kepler Telescope catalog for his research, Niraula came upon a signal from Kepler-854b that appeared too large to be true.
“Suddenly we had a system where we saw this ellipsoidal signal which was huge, and pretty immediately we knew this could not be from a planet,” says Shporer. “Then we thought, something doesn’t add up.”
With their new-found information, researchers looked at both the star and planetary candidate. Kepler-854b was spotted through a transit detection in 2016. Using more accurate measurements from the European Space Agency’s Gaia mission, Niraula found Kepler-854b to be three times the size of Jupiter.
“There’s no way the universe can make a planet of that size. It just doesn’t exist,” states Shporer.
The MIT team then wondered if there could be more misclassified planets. They poured through the Kepler catalog’s of more than 2,000 planets and, using the Gaia measurements, they discovered three more planets that are actually stars, since they are two to four times Jupiter’s size.
“That was a very big flag. We now have three objects that are now not planets, and the fourth is likely not a planet,” says Niraula.
Researchers don’t believe there won’t be many more corrections to existing exoplanets going forward.
“This is a tiny correction,” says Shporer. “It comes from the better understanding of stars, which is only improving all the time. So, the chances of a star’s radius being so incorrect are much smaller. These misclassifications are not going to happen many times more.”
The study is published in the Astronomical Journal.