An article published in “The Astronomical Journal” reports a study that reviews the data of four objects discovered a few years ago thanks to NASA’s Kepler space telescope, concluding that at least three of them are actually tiny stars and not planets. A team of researchers used new data collected by ESA’s Gaia space probe to get more precise information on objects cataloged as exoplanets. Kepler-854b, Kepler-840b, and Kepler-699b appear to have sizes between two and four times Jupiter’s, too much even for hot Jupiters very close to their stars but possible for tiny stars. Kepler-747b turns out to have a radius 1.8 times Jupiter’s even though it’s quite far from a star a little smaller than the Sun, so its nature is uncertain.
NASA’s Kepler Space Telescope has been an extraordinary planet hunter for years. Its mission ended in October 2018 but it allowed the discovery of a few thousand exoplanets. However, every exoplanet candidate requires a follow-up study first of all to confirm its existence and even more so to have precise estimates regarding its orbit, mass, and size.
The results of the studies of the exoplanets discovered improved over time thanks to more refined computing systems but also thanks to other sources of information. The Gaia space probe is a special space telescope designed by ESA to map space objects by collecting accurate data about them. This offered the possibility of re-examining already known exoplanets.
The authors of this new study were looking in the Kepler Space Telescope’s archive for potential systems with traces of tidal distortion. Their research led them to discover exoplanets that, from the data collected by the Gaia space probe, are too large to be planets, even for hot Jupiters very close to their stars. In particular, Kepler-854b and Kepler-840b have a year that lasts just over two Earth days, so rough estimates of their size could indicate that they were exoplanets inflated by the intense heat they receive. Updated information makes this interpretation unlikely.
Now the exoplanet section of the NASA website indicates that Kepler-854b (seen in an artistic concept as a super-Jupiter planet in the image compared to Jupiter) and Kepler-840b have masses estimated to be over 130 times Jupiter’s. This suggests a stellar nature even more than their size much higher than Jupiter’s. The question is more complex for Kepler-699b, for which an estimate of the mass is missing but from Gaia’s data, it appears to have a radius 2.76 times Jupiter’s. It’s too large an object to be a planet, also because it’s far farther from its star than the other two objects. According to the researchers, Kepler-699b’s characteristics indicate that it can’t be a brown dwarf, an object between planet and star, but is likely a tiny star as well.
Kepler-747b’s examination yielded more ambiguous results. The new estimate of the radius is 1.8 times Jupiter’s but on the NASA database now its mass is indicated at about 24 times Jupiter’s, too little to be a star. The information is still inaccurate, so its nature remains uncertain. Further observations could provide the data necessary for more precise estimates to understand whether it’s a brown dwarf or its mass is actually much higher and therefore it’s a star as well.
Ultra-cool dwarf stars are very dim and their light can be mistaken for that of a hot Jupiter. After more than twenty years of exoplanet studies, there are still cases where the information is not sufficient to accurately estimate their characteristics. These cases are now rare because the data collected have increased over time and their processing has improved, proving how far this branch of astronomy has progressed.
