Among the thousands of known exoplanets, astronomers at the Massachusetts Institute of Technology have identified three stars that are actually stars. Pictured is an artist’s interpretation of stars and planets. credit: NASA
Among the thousands of known exoplanets,[{” attribute=””>MIT astronomers flag three that are actually stars.
The first worlds beyond our solar system were discovered three decades ago. Since then, close to 5,000 exoplanets have been confirmed in our galaxy. Astronomers have detected another 5,000 planetary candidates — objects that might be planets but have yet to be confirmed. Now, the list of planets has shrunk by at least three.
In a study published on March 15, 2022, in the Astronomical Journal, MIT astronomers report that three, and potentially four, planets that were originally discovered by NASA’s Kepler Space Telescope are in fact misclassified. Instead, these suspected planets are likely small stars.
The team used updated measurements of planet-hosting stars to double-check the size of the planets, and identified three that are simply too big to be planets. With new and better estimates of stellar properties, the researchers found that the three objects, which are known as Kepler-854b, Kepler-840b, and Kepler-699b, are now 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. “Bigger than that couldn’t be a planet, and that’s what we found,” says study first author Prajwal Niraula, a graduate student in the Department of Earth, Atmospheric and Planetary Sciences at MIT.
The fourth planet, Kepler-747b, is about 1.8 times the size of Jupiter, which is comparable to the largest of the confirmed planets. But Kepler-747b is relatively far from its star, and the amount of light it receives is too small to sustain a planet its size. The team concluded that the state of Kepler 747b is questionable but not entirely implausible.
“Overall, this study makes the current list of planets more complete,” says study author Avi Shporer, a research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. People rely on this list to study the planet population as a whole. If you are using a sample with a few hackers, your results may be inaccurate. So, it is important that the list of planets is not polluted.”
Also among the study’s co-authors are Ian Wong, a NASA Postdoctoral Fellow at NASA’s Goddard Space Flight Center, and MIT Associate Professor Julian De Wit.
Excellent updates
Eliminating the Planetary Impostors was not the team’s initial goal. Niraula was originally intended to search for systems with signs of tidal deformation.
“If you have two bodies that are close together, the attraction of one will cause the other to be egg-shaped, or ellipsoid, which gives you an idea of how big this companion is,” explains Niraula. “So you can determine if it’s a star system or a star planet, based on that tidal clouds.”
While combing through the Kepler catalog, he found a signal from Kepler-854b that seemed too big to be true.
“All of a sudden we had a system where we saw this elliptical signal that was huge, and right away we knew this couldn’t be from a planet,” says Sporer. “Then we thought something didn’t add up.”
The team then took a second look at the star and planetary filter. As with all planets discovered by Kepler, Kepler-854b was observed through the discovery of transits – a periodic dip in starlight that indicates the possibility of a planet passing in front of its star. The depth of this drop represents the ratio between the size of a planet and the size of its star. Astronomers can calculate the size of a planet based on what they know about the size of the star. But with the discovery of Kepler-854b in 2016, its size was based on stellar estimates that were less accurate than they are today.
Currently, the most accurate measurements of stars come from the European Space Agency’s Gaia mission, a space observatory designed to accurately measure the properties and trajectories of stars on a world map.[{” attribute=””>Milky Way. In 2016, Gaia’s measurements of Kepler-854 were not yet available. Given the stellar information that was available, the object seemed to be a plausible-sized planet. But Niraula found that with Gaia’s improved estimates, Kepler-854b turned out to be much larger, at three times the size of Jupiter.
“There’s no way the universe can make a planet of that size,” Shporer says. “It just doesn’t exist.”
Tiny corrections
The team confirmed that Kepler-854b was a planetary “false positive” — not a planet at all, but instead, a small star orbiting a larger host star. Then they wondered: Could there be more?
Niraula searched through the Kepler catalog’s more than 2,000 planets, this time for significant updates to the size of stars provided by Gaia. He ultimately discovered three stars whose sizes significantly changed based on Gaia’s improved measurements. From these estimates, the team recalculated the size of the planets orbiting each star, and found them to be about two to four times Jupiter’s size.
“That was a very big flag,” Niraula says. “We now have three objects that are now not planets, and the fourth is likely not a planet.”
Going forward, the team anticipates that there won’t be many more such corrections to existing exoplanet catalogs.
“This is a tiny correction,” Shporer says. “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.”
Reference: “Revisiting Kepler Transiting Systems: Unvetting Planets and Constraining Relationships among Harmonics in Phase Curves” by Prajwal Niraula, Avi Shporer, Ian Wong and Julien de Wit, 15 March 2022, Astronomical Journal.
DOI: 10.3847/1538-3881/ac4f64
This research was supported in part by NASA.