Three “Exoplanets” Turn Out To Be Stars

Stars and planets illustration

Among the thousands of known exoplanets, MIT astronomers have identified three that are actually stars. The photo shows an artistic interpretation of the 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. The planet couldn’t be bigger than this, which is what we found,” says study lead 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, comparable to the largest confirmed planets. But Kepler-747b is relatively far from its star, and the amount of light it receives is too small to support a planet this size. The researchers concluded that the status of the planet Kepler-747b is dubious, but not entirely implausible.

“Overall, this study makes the current list of planets more complete,” says study author Avi Sporer, a research fellow at MIT’s Kavli Institute for Astrophysics and Space Studies. “People rely on this list to study planetary populations in general. If you use a sample with multiple offenders, your results may not be accurate. Therefore, it is important that the list of planets is not polluted.”

Study co-authors also include Jan Wong, a NASA scientist at the Goddard Space Flight Center, and MIT assistant professor Julien de Wit.

Star Updates

The eradication of planetary impostors was not the team’s original goal. Nyraula’s original intention was to look for systems with signs of tidal distortion.

“If you have two objects close together, the gravitational pull of one of them will make the other egg-shaped or ellipsoidal, giving you an idea of ​​how massive the companion is,” Niraula explains. “So you can tell if it’s a star-star or star-planet system just based on that tidal pull.”

Looking through the Kepler catalog, he came across a signal from Kepler-854b that seemed too big to be true.

“Suddenly, we had a system where we saw this ellipsoidal signal, which was huge, and almost immediately we knew it couldn’t be a planet,” Sporer says. “Then we thought something didn’t add up.”

The team then took another look at both the star and the planetary candidate. Like all planets discovered by Kepler, Kepler-854b was discovered using transit detection, a periodic dip in starlight that signals a planet’s possible transit in front of its star. The depth of this dip is the ratio between the size of the planet and its star. Astronomers can calculate the size of a planet based on what they know about the size of a star. But since Kepler-854b was discovered in 2016, its size was based on stellar estimates that were less accurate than today.

Currently, the most accurate measurements of stars are made by the European Space Agency’s Gaia mission, a space-based observatory designed to accurately measure and map the properties and paths of stars in space.[{” 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.

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