With the launch of NASA’s TESS satellite due this very day, this is a popular-level account of TESS and exoplanet hunting that I wrote for The Conversation (and which has been re-published by the BBC Focus Magazine). Actually this is my version, prior to their editing.
Previous generations have looked up at the stars in the night sky and wondered whether they are also orbited by planets; our generation is the first to find out the answer. We now know that nearly all stars have planets around them, and as our technology improves we keep finding more. NASA’s newest satellite, TESS (the Transiting Exoplanet Survey Satellite), scheduled for launch on Monday, will extend the hunt for small, rocky planets around nearby, bright stars.
We want to know how big such planets are, what orbits they are in, and how they formed and evolved. Do they have atmospheres, are they clear or cloudy, and what are they made of? It is conceivable that, over coming decades, we might find a planet that has a size and mass similar to Earth’s. If that planet also lies at the right distance from its star for water to be liquid, and has an atmosphere containing molecules such as free oxygen and has an atmosphere containing molecules such as free oxygen then we could even find biological activity. TESS is a major step towards that long-term goal.
Planets are so faint and tiny compared to their host stars that it is remarkable we can detect them at all, let alone study their atmospheres. TESS exploits the fact that, if the planet’s orbit is edge on to us, then it will “transit” across the face of the star, blocking a small fraction of the star’s light. TESS will monitor 200,000 bright stars in the solar neighbourhood, looking for tiny dips in their brightness that reveal a transiting planet.
During transit, the planet’s disc is surrounded by the thin smear of its atmosphere, back-lit by the bright star. That starlight will be absorbed at some wavelengths by molecules in the planet’s atmosphere, while at other wavelengths it shines straight through. By observing the starlight passing through the planet’s atmosphere and splitting it up into its spectrum, we can deduce what that atmosphere is made of.
Such observations are right at the limit of current capabilities, requiring the James Webb Space Telescope (JWST), the $8 billion successor to Hubble scheduled for launch in 2020. With a 6.5-m mirror, collecting much more light than Hubble, and with specially designed instruments, JWST has been built to study exoplanet atmospheres.
With JWST time being so valuable we first need to know which stars have the best transiting exoplanets to study, and that’s why we need TESS. Its predecessor spacecraft, Kepler, surveyed 150,000 stars in a patch of sky near the constellation Cygnus, and found over a thousand planets ranging from gaseous giants like Jupiter to rocky planets as small as Mercury. But Kepler covered only a small patch of sky, containing few bright stars, so most of the stars it looked at are faint, and that makes it hard to study their planets.
In contrast, ground-based surveys for transiting exoplanets have searched wider swathes of the sky looking at brighter stars. The most successful has been the UK-led Wide Angle Search for Planets, of which I am a member. Using an array of camera lenses, the WASP project has spent the last decade monitoring a million stars every clear night looking for transit dips. Nearly two hundred planets have been found, and indeed many of the transiting exoplanets so far chosen as JWST targets come from WASP.
But ground-based transit surveys have one big limitation: they look through Earth’s atmosphere, and that severely limits the data quality. They can detect brightness dips of 1 per cent, which is sufficient to find giant gaseous planets like our own Jupiter and Saturn, but smaller, rocky planets block out far less light. Our Earth would produce a dip of only 0.01 per cent if seen projected against our Sun.
TESS will combine the best of both, observing bright stars over the whole sky, but with the advantage of space-based photometric quality. It should find the small, rocky planets that Kepler proved are abundant, but transiting much brighter stars. That means they can be studied and characterised by JWST.
TESS will observe each region of sky for typically 30 days, meaning that it will be most sensitive to planets with short orbital periods that produce frequent transits. Planets in short-period orbits are close to their star. Therefore most of the planets that TESS finds will be highly irradiated and too hot for liquid water. But if found orbiting red-dwarf stars, which are intrinsically dim, even close-in planets can be in the habitable zone.
The dwarf star TRAPPIST-1 is a thousand times less luminous than our Sun, and is known to host seven short-period planets. TESS will look for such planets around other dwarf stars, and while TESS does so from space, the SPECULOOS survey will be attempting the same from the ground. All such finds will be prime targets for JWST.
Looking further ahead we then want to find rocky planets in the habitable zone of stars like our Sun. They will have longer-period orbits and so will be harder to find. That is the task of the ESA satellite PLATO, currently under development for launch in 2026. The race will then be on to find biomarker molecules in the atmosphere of an Earth-twin exoplanet.