TESS goes above and beyond
NASA’s Transiting Exoplanet Survey Satellite, also known simply as TESS, has been spectacularly successful, and two recent discoveries of exoplanets − planets in orbit around other stars − are important examples.
TESS was launched into space in April 2018. Its ovalshaped orbit causes its distance from Earth to vary between 108,400km and 376,300km. It’s an orbit that will remain very stable.
As its name suggests, TESS searches for exoplanets by watching the slight dimming that is produced by planets moving in front of their ‘parent’ stars as seen from Earth. This passage is called a transit, and it’s a method that has been used by the Kepler spacecraft and many Earth-based planet searches.
TESS is especially intended for the study of planets belonging to the brightest stars and, importantly, stars relatively close to our Sun.
As I mentioned in an article a year ago this weekend, TESS began with the discovery of eight confirmed exoplanets, and it has continued on to find many more. However, two recent discoveries stand out.
Firstly, TESS has detected its first Earth-sized planet, called TOI 700d, in the habitable zone around its parent star. This zone is the range of distances from the stars in which water can exist as a liquid. This range depends, of course, on the star itself, with the less energetic stars having habitable zones that are closer in to the star.
The star is a red dwarf star in the far southern sky. It is in the constellation of Dorado, The Swordfish, and is about a hundred light years away from us. This type of star is about half the diameter of our Sun, and is rather cooler than the Sun, meaning that its habitable zone is closer in to the star than the equivalent zone around our Sun.
Although it is not the first such planet to be discovered, it is certainly TESS’s first, and there is a delightful story associated with this work. NASA has pointed out that the star was originally classified incorrectly in their database, leading astronomers to produce erroneous results for the planets moving around it. The wrong information suggested that the star was more like our Sun, which meant that the proportional drop in light initially indicated that its planets were larger.
That this is not the case was discovered when some of the TESS researchers noticed the error, and this group of people included a high school student, Alton Spencer, working with the team. Well done to Alton for his input.
Secondly, TESS has made its first discovery of a planet moving around a binary star system − only the 13th known such planet. In this scenario, there are two stars orbiting each other, and a planet orbiting the pair at a distance so that an observer on the planet would see two ‘suns’ in the sky.
This type of scene has been used in science fiction stories, notably Star Wars in which the planet Tatooine orbits a binary star pair.
The binary system under study for TESS is TOI 1338, also in the southern sky. In this case, it is in the direction of the constellation of Pictor, The Painter, adjacent to Dorado. The two stars orbit each other about every 15 days, and the planet moves around the pair in about 94 days.
Observing the slight drops in light caused by the planet passing between us and a star is particularly interesting in binary systems. In this case, the planet ‘transits’ only one of the two stars as seen from here, but the period between each transit varies between about 93 and 95 days because the stars themselves are in motion around one another. So after each transit, the star will be in a different place next time around.
I think it would be fascinating to watch these stars from the planet, repeating their dance around each other every 15 days.