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Look! Up in the air! Is it a world? Nope, just a single star.


Three decades ago, the first planets identified outside our solar system were discovered. Since then, about 5,000 exoplanets have been discovered in our galaxy. Another 5,000 planetary candidates have been discovered by astronomers, which are objects that might be planets but have yet to be verified. The number of planets has now been reduced by at least three.

In a research published in the Astronomical Journal, MIT astronomers reveal that three, and perhaps four, planets identified by NASA’s Kepler Space Telescope have been misclassified. These putative planets are most likely tiny stars.

The researchers utilized new measurements of planet-hosting stars to double-check the size of the planets and discovered three that are just too large to be planets. With fresh and improved stellar property estimations, the researchers discovered that the three objects, known as Kepler-854b, Kepler-840b, and Kepler-699b, are now estimated to be two to four times the size of Jupiter.

“The majority of exoplanets are the size of Jupiter or considerably smaller. Twice the size of Jupiter already raises suspicions. We discovered that anything larger than that cannot be a planet “Prajwal Niraula, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences, is the study’s first author.

Kepler-747b, a fourth planet, is around 1.8 times the size of Jupiter and is equivalent to the most biggest verified planets. However, Kepler-747b is a long way from its star, and the quantity of light it gets is insufficient to maintain a planet of its size. The team thinks that Kepler-747b’s planetary status is questionable but not wholly improbable.

“Overall, our study adds to the existing list of planets,” study author Avi Shporer, a research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, said. “This list is used to examine the population of planets as a whole. Your findings may be erroneous if you utilize a sample containing a few interlopers. As a result, it is critical that the list of planets be not tainted.”

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Co-authors of the paper include Ian Wong, a NASA Postdoctoral Program Fellow at NASA Goddard Space Flight Center, and Julien de Wit, an assistant professor at MIT.

Exceptional updates

The team’s first purpose was not to find planetary imposters. Niraula’s first plan was to hunt for systems that showed symptoms of tidal distortion.

“If two objects are near to one other, the gravitational attraction of one will force the other to be egg-shaped or ellipsoidal, which gives you a sense of how massive the companion is,” Niraula adds. “So you could tell whether it’s a star-star or star-planet system simply by looking at the tidal force.”

He stumbled upon a signal from Kepler-854b that looked to be too massive to be genuine when looking through the Kepler database.

“All of a sudden, we had a system where we observed this ellipsoidal signal that was massive, and we understood right away that this couldn’t be from a planet,” Shporer adds. “Then we realized that something doesn’t add up.”

The researchers then revisited both the star and the planetary candidate. Kepler-854b, like other Kepler-detected planets, was discovered by a transit detection, which is a periodic drop in sunlight that indicates a probable planet passing in front of its star. The depth of the dip shows the ratio of the planet’s size to that of its star. Astronomers can compute the size of the planet based on what they know about the size of the star. However, when Kepler-854b was identified in 2016, its size was based on stellar calculations that were not as exact as they are now.

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The European Space Agency’s Gaia project, a space-based observatory intended to accurately measure and record the attributes and motions of stars in the Milky Way, currently provides the most accurate measurements of stars. Kepler-854 observations from Gaia were not yet available in 2016. Given the existing stellar data, the object seemed to be a plausible-sized planet. But, according to Gaia’s revised calculations, Niraula discovered that Kepler-854b is three times the size of Jupiter.

“There’s no way the cosmos could create a planet that big,” Shporer argues. “It just does not exist.”

Minor alterations

The scientists determined that Kepler-854b was a planetary “false positive” – a tiny star circling a bigger host star rather than a planet. Then they pondered whether there may be more.

Niraula combed through the Kepler catalog’s over 2,000 planets, looking for substantial revisions to the size of stars reported by Gaia. He eventually located three stars whose diameters had altered dramatically as a result of Gaia’s enhanced observations. The scientists estimated the sizes of the planets around each star based on these assumptions and discovered that they are about two to four times the size of Jupiter.

“That was a large flag,” Niraula remarks. “Three objects are no longer planets, and the fourth is most certainly not a planet.”

The team believes that there will be few more similar revisions to current exoplanet databases in the future.

“This is a little change,” Shporer explains. “It stems from a greater knowledge of stars, which is always evolving. As a result, the odds of a star’s radius being so off are substantially lower. These misclassifications will not occur often in the future.”

NASA helped fund some of this study.

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