TESS:

The planet hunt

or the last two years, the Transiting Exoplanet Survey Satellite (TESS) has been watching the stars, searching for new worlds. This month it completes its primary mission, having scanned 70 per cent of the sky – an area 400 times larger than that covered by its predecessor, the Kepler space telescope; but its job is far from done. As TESS moves into a two-year extension, we take a look over what the mission has already achieved and what it still could.

Like Kepler, TESS has been tracking some 200,000 stars, watching out for the tell-tale dip in brightness as a planet passes – or transits – in front of the star. “TESS’s particular niche is that it’s sensitive to red dwarf stars,” says Sara Seager, deputy science director for TESS. “The reason for that is that we can find small planets transiting small stars more easily than small planets transiting larger stars. The dream is to find small rocky planets in the habitable zones of their stars.”

The ultimate goal is to find 50 new small planets with accurate size and mass measurements, which the TESS team will then distribute to the world. “From TESS itself you just get the planet’s size and the orbital period,” says Seager. “You can do follow up mass measurements of planets, which takes a very long time and for a lot of them we can’t. People are busy finding planets.”

At the time of writing, TESS had identified 1,835 planetary candidates. To investigate these further, the project has a team of around 500 individuals from 100 institutions doing follow-up observations.

FOvercoming obstacles

The first step of this follow-up is confirming the planet is actually a planet. The TESS team take very precise images of the host star to make sure it’s not a pair of close binary stars, which can produce similar patterns of rising and falling brightness.

Normally these observations would happen within a few months of TESS discovering a candidate, but the global COVID-19 pandemic has forced many of the telescopes that would do these observations to close their domes. “The follow-up has really stalled,” says Seager. “Objects are only favourably placed in the sky for a short amount