TESS the Exoplanet Hunter Makes "Totally Unexpected" Third Discovery

When NASA’s Kepler mission ended in October, the proverbial planet-finding torch was passed to TESS, the Transiting Exoplanet Survey Satellite. On Monday, TESS’s team announced at the American Astronomical Society winter meeting that the intrepid satellite discovered its third small planet outside of our solar system: a gaseous planet about three times the size of Earth.

Named HD 21749b, this planet orbits a dwarf star about 53 light-years away in the constellation Reticulum. TESS found it by monitoring the sky in chunks, looking for momentary dips in light. So far, it’s examined the first three of 13 sectors that make up the southern sky — and it was within sector one that scientists observed a single dip in light from star HD 21749.

What exoplanet hunters want, study co-author Johanna Teske, Ph.D. tells Inverse, are those dips in light. Dips signify that something is blocking out part of the star’s light — it doesn’t necessarily mean there is a planet, but changes in a stars brightness signify that something is going on, hinting that it’s time to investigate further.

To corroborate this dip, Teske and her colleagues looked through data previously collected by HARPS, a high-precision spectrograph on a ground-based telescope in Chile, which showed that a signal emanates from the star HD 21749 every 36 days. Comparing that pattern to the full transit discovered by TESS, the team discovered that those 36 days corresponded with the orbit of a planet.

The first light images from TESS, showing the combined view of all four of its cameras.

Further ground observations validated their findings and helped the team determine the mass, bulk density, and nature of the planet. HD 21749b, while larger than Earth, is much smaller than previously discovered exoplanets and, while its surface is at 300 degrees Fahrenheit, it’s still relatively cool given its proximity to its very bright star. Its smallness and relative coolness are key because the heart of TESS’s mission is finding another Earth.

“We’re motivated by finding something like Earth, but along the way we are finding all these different kinds of planets that are totally unexpected and unlike what’s in our solar system,” Teske, a Hubble postdoctoral fellow at the Carnegie Institution for Science, explains. “Even now, after thousands of exoplanets have been discovered, there are always surprises.”

Diana Dragomir Ph.D., the MIT postdoctoral scholar who led this study, tells Inverse that the discovery of this cool planet means scientists are “pushing towards an understanding of the atmosphere of cooler planets and moves us towards, eventually, finding Earth-temperature planets.” She’s interested in continuing her studies on HD 21749b, but, as this is just the third small planet TESS has found, she’s raring to find more. Dragomir explains that “the more planets we can study, the more we can determine how frequent they are.”

She predicts that, by the end of TESS’ two-year mission scanning the 20 million stars that make up the entire night sky, they will have found an enormous host of planets. The discovery of the next planet, meanwhile, may happen sooner rather than later: Dragomir says that the team has also detected evidence of a potential second planet in the same planetary system. Its planetary status still needs to be confirmed, but it’s shaping up to look Earth-sized — but much, much hotter than the planet we call home.