TESS Discovers its 3rd New Planet

The Transiting Exoplanet Survey Satellite (TESS) of NASA found a dense, gaseous world having an orbital period of 36 days around a bright dwarf star.

TESS found a new exoplanet around a star which is about 53 light-years away from us. The name given to this new planet is HD 21749b and it orbits a dwarf star in the constellation Reticulum. It is the 3^rd planet which has been identified by TESS so far and it offers the longest orbital period (36 days) among all of them. The other two planets that were discovered by TESS are Pi Mensae b and LHS 33844b. The latter of the two takes just 11 hours to orbit its star while Pi Mensae b has a 6.3-day orbit. Surprisingly, the surface of HD 21749b is quite cool (around 300^o F), given its proximity to its star which is almost as hot as our sun. Diana Dragomir, the Team Lead of the discovery from the Kavli Institute for Astrophysics and Space Research, referred to that and said,

“It’s the coolest small planet that we know of around a star this bright. We know a lot about atmospheres of hot planets, but because it’s very hard to find small planets that orbit farther from their stars, and are therefore cooler, we haven’t been able to learn much about these smaller, cooler planets. But here we were lucky, and caught this one, and can now study it in more detail.”

HD21749b is placed in the ‘sub-Neptune’ category because it is three times the size of Earth and is made up of gas. Researchers mentioned that it is highly unlikely for this planet to be habitable as it doesn’t appear to have a rocky core. It is primarily composed of a gas, just like Uranus or Neptune, but the density of the gas is much higher in case of HD21749b. Dragomir acknowledged that and suggested that the planet might have a thick atmosphere in comparison to Uranus and Neptune, who are mostly made up of Hydrogen.

TESS uses the ‘Transit Method’ to look for planets as it monitors the sky, sector by sector. It spends 27 days in each sector and tries to locate exoplanets by observing any momentary dips in the light of nearby stars through its four cameras. It is expected that TESS will explore the entire sky during its 2-year mission. It is currently surveying the sky in the Southern Hemisphere and will continue to do for the complete first year of its operation. After that, it will turn its attention to the sky of Northern Hemisphere.

There are a total of 13 sectors in the Southern Hemisphere that the TESS will survey. During the analysis of the data gathered in sector 1, Dragomir noticed a single transit in the light from HD 21749. As we know that TESS only stays in a sector for 27 days, it becomes difficult to identify exoplanets which have a longer orbital period than that (which was the case with HD 21749b). What made things even more problematic for the researchers was that the star itself is quite active. For this reason, Dragomir decided to consult the dataset gathered by the High Accuracy Radial velocity Planet Searcher (HARPS). This high-precision spectrograph identifies exoplanets through their gravitational tug on their host stars. Dragomir explained that experience in the following words:

“They had looked at this star system a decade ago and never announced anything because they weren’t sure if they were looking at a planet versus the activity of the star. But we had this one transit, and knew something was there.”

The data from the HARP revealed that a repeating signal was emanating from HD 21749 every 36 days. This urged the researchers to have a look at the data collected by TESS during the exploration of sector 3. Unfortunately, a momentary glitch created a gap in the data exactly at the point when Dragomir expected the second transit to happen. However, she didn’t lose hope and gave it another try by re-extracting the data. She described that by saying,

“Because there was an interruption in data around that time, we initially didn’t see a second transit, and were pretty disappointed. But we re-extracted the data and zoomed in to look more carefully, and found what looked like the end of a transit.”

They also compared this pattern with the first full transit and got almost a perfect match. In addition to that, they validated their findings by using the data from the Planet Finder Spectrograph on the Magellan Telescope in Chile. The mission team of TESS aims to provide information on 50 small exoplanets by the end of its 2-year mission.