It’s time for other missions and projects to pick their favourite planets and take a closer look
of time. It could delay confirming planet candidates for five months.”
Once confirmed, astronomers work out the size of the planet by measuring how much the star’s brightness goes down during the transit – the bigger the dip, the bigger the planet. These planets appear to range in size from around the radius of Earth to several times the width of Jupiter, with the most common type being mini-Neptunes – planets around two to three times the size of Earth. “These have no Solar System counterpart,” says Seager. “We don’t know what they are. Are they all different? Are they just one type of planet? TESS has been really successful in finding those around both Sun-like and dwarf stars.”
The size distribution also backs up something that was found by the Kepler space telescope – there is a curious lack of planets between 1.5 and 2 Earth radii, a range known as the Fulton gap. “TESS has found planetary systems on either side of that gap,” says Seager. It’s thought the key to the gap lies in the planet’s atmospheres – stellar wind and radiation strips away the gases from the smaller planets, while the upper edge marks the point where planets are large enough to hold onto their atmospheres. “In the future we hope to look at their atmospheres with other telescopes and find why the gap appears to exist and try to make sense of it,” says Seager.
A precise approach
The final stage of TESS’s follow-up is to determine the masses of planets. Doing this requires precise spectroscopy to measure the stellar wobble caused by the planet pulling on the star. “Those are very specific instruments and there are only a handful of them around the world,” says George Ricker, TESS’s principal investigator. “We’re getting close at this point to having the 50 measured masses.”
Once the team have both size and mass in hand it’s possible to work out the density of the planet, giving
The extended TESS mission will aim to fill in gaps (black regions) left by the initial observing run, which will cross over many regions observed by Kepler
KEPLER
KEPLER K2 FIELDS an indication of whether it’s rocky or gaseous. At this point, the TESS team’s job is largely done; it’s time for other missions and projects to pick their favourite planets and take a closer look.
One of the forefront areas of exoplanet research at the moment is analysing these alien atmospheres in detail. By looking for key chemicals in a planet’s atmosphere, such as oxygen, carbon dioxide or water, astronomers can begin to understand what these planets are like, which is particularly important for planets like mini-Neptunes which have no counterpart in the Solar System. This in turn helps planetary
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scientists to craft more complete theories about how planets grow and change to create the planetary systems we see today.
One particular area that always draws attention is hunting down planets which might show signs of life, known as biosignatures. Mostly, these are chemical imbalances caused by the presence of bacteria or plants, but TESS has also partnered up with the Breakthrough Listen project to search promising planets for ‘technosignatures’ – radio signals from an advanced civilisation.
The possibilities of TESS go far beyond merely looking for exoplanets, however. As it was observing the stars, TESS’s four cameras were also taking wide-field images of the sky. Unlike most projects, which jealously guard their data for a few months, all of TESS’s data is made publicly available as soon as it’s ready. “That’s an aspect of why TESS has been so successful,” says Ricker. “We’ve been able to instigate this feeling of collaboration rather than competition.”
TESS observes the sky in 24 x 90˚ sections – about four times the size of Orion – taking images every 30 minutes or so. These kinds of observations are perfect for spotting transient events, such as supernovae, and TESS finds around 100 of these every year.
Looking ahead
Yet despite this wide coverage, TESS’s primary mission only observed around 70 per cent of the sky. As the mission now heads into its extension the spacecraft will aim to fill in these gaps. Notably, the satellite will cover the ecliptic plane, though these observations will be looking at worlds a little closer to home. “There’s a lot of Solar System science that we’re going to do,” says Ricker. “TESS is able to detect some 100,000 asteroids, Kuiper Belt objects and trans-Neptunian objects.”
The extension will also allow TESS to go back over areas which were only briefly observed, allowing astronomers the chance to uncover planets further from their stars with a longer wait between successive transits. “We should be able to find many more habitable-zone planets on which liquid water might be able to exist,” says Ricker. “For something like Earth, you’d need to observe the Sun for a year to see a transit and that’s why this extended mission is going to be really important.”
The latest mission extension will last until 2022, but the TESS team have plans long beyond that. “We hope to carry on operations to 2028 as the satellite is operating really well. We think we’ll be able to observe for at least a decade, hopefully a lot more,” says Ricker.
The primary mission of TESS might now be complete, but its task is far from finished.