Didn't she do well? NASA's TESS reveals new exoplanets

2019 brings us newly discovered planets thanks to NASA's Transiting Exoplanet Survey Satellite

By Adam Dorey

January may seem cold and far away now but even colder and further away is NASA’s Transiting Exoplanet Survey Satellite (TESS). From its elliptical orbit around the Earth, TESS is searching for planets outside our solar system, referred to as exoplanets. At least eight exoplanets have been discovered, with details on another 20 to 30 planets nearing publication.

On January 7th, a new exoplanet discovery was announced – but the catchily named HD 21749b, which orbits the star HD 21749, has piqued scientific curiosity for three reasons. First, the planet is surprisingly cool, no similarly bright star hosts a planet as cool. While scientists have a better understanding of planets with hotter atmospheres, this cooler planet offers an insight into a lesser known atmosphere. Second, evidence suggests a smaller, Earth-like planet also orbits the host star. If confirmed, it would be the first Earth-sized planet discovered by TESS. Third, the two different planets form a peculiar system. Scientists don’t understand how the larger planet’s long, elliptical orbit can occur alongside the smaller planet’s – the gravitational pull of the larger planet should disrupt the smaller planet’s orbit. Researcher Xu Chelsea Huang, who presented the findings, claims ‘this is the most extreme system with this type of architecture.’

In fact, TESS’ discovery of an exoplanet forming a similar, mystifying system was amongst its first findings. Scientists already knew of a Jupiter-like (but ten times larger) planet that orbits a star called Pi Mensae. Its orbit is very eccentric – at some points it is further away than Jupiter is from our own Sun, but at others it intrudes into the star’s habitable zone. This zone is where liquid water can exist on the planet without evaporating or freezing; Earth falls within the habitable zone of our Sun. TESS found a smaller exoplanet extremely close to Pi Mensae – another example of a small planet and a large planet coexisting with puzzling orbits. Without TESS, researchers couldn’t have known this odd planet was actually part of an even odder planetary system existing right under our noses.

On a two-year mission, TESS is scouring the orbits of some 200,000 nearby main-sequence stars – these are stars that generate heat through nuclear fusion of hydrogen into helium, like our Sun. TESS uses the transit method: it looks for repeated, periodic decreases in the visible light of a star, indicating that a planet is passing in front of it. After follow-up investigations confirm discovered planets are truly there (and not false positives), researchers measure their size and orbit using TESS and measure the planet’s mass using a ground-based telescope. This information will help to determine the planets’ compositions and atmospheres, filling in intricate details of worlds above. Essentially, TESS shines a beacon into the unknown by measuring what shines back.

The details TESS receives allows scientists to understand planetary system formation and which types of planet appear around stars of different ages – information that could predict the future of planetary systems that may one day harbour life. Finding planets that might support life is one of NASA’s goals for its exoplanet exploration. We can calculate which planets live in the habitable zones around stars from orbit data, we can detect gases that could only be produced biologically through studying atmospheric data. With the first TESS conference scheduled to begin on July 29th and data pouring in, it seems likely our knowledge of other worlds will lead us to further scientific discovery – whether learning more about our endless universe or even discovering other forms of life.

From Issue 18