Three Kepler Exoplanets May Actually Be Tiny Stars

When exoplanets come up, we’re usually adding to the total number of known worlds beyond our solar system, but not today. MIT astronomers applied the latest astronomical data to Kepler’s archive of exoplanets, finding several that no longer add up. According to the study published in the Astronomical Journal, there are at least three exoplanets among Kepler’s 5,000 finds that are actually low-mass stars.

Identifying exoplanets from light years away is no simple feat because we can’t just look at them directly. A planet like Earth or even Jupiter is too dim to be visible next to a much brighter star, so Kepler and most other exoplanet hunters like TESS rely on transits. They watch for repeating dips in luminance of stars that could indicate a planet has passed in front. With some basic details like the location and size of the host star, we can confirm (or not) the presence of an exoplanet.

The MIT team didn’t set out to un-confirm exoplanets — the project started as a search for evidence of tidal distortion in exoplanets. However, lead author Prajwal Niraula added the latest data from the ESA’s Gaia mission, which offers the best available catalog of star location and size. With better data than we had during Kepler’s mission, some of the alleged exoplanets ballooned in size.

A planet called Kepler-854b captured Niraula’s attention right away. It’s not unusual to find exoplanets larger than Jupuiter, but our local gas giant is still pretty large in the grand scheme. The largest confirmed exoplanets are about 1.7 times the diameter of Jupiter, but planetary status is increasingly suspect as you get to double Jovian scale. That’s about where Kepler-854b fell once the Gaia data was added. It’s more likely Kepler-854b is a brown dwarf, sometimes called a failed star. These objects are more massive than planets, but they don’t have enough mass to sustain hydrogen fusion. Instead, they can only fuse larger elements like lithium.

Wondering if there were more not-planets hiding in the Kepler archive, Niraula search through 2,000 more planets, finding two more worlds (Kepler-840b and Kepler-699b) that were clearly on the wrong side of the planet-star division, clocking in at between two and four times the size of Jupiter. A fourth planet is also suspect at 1.8 times larger than Jupiter. That’s not impossible, but it would make Kepler-747b one of the largest known exoplanets.

This change does not mean our method of spotting exoplanets is wrong — it’s the best we can do until more powerful telescopes are available. However, accurate stellar data is key to correctly interpreting the twinkling of stars.

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