TESS hits the trifecta: Nearby bright star has 3 interesting planetsThanks to the massive trove of exoplanets discovered by the Kepler mission, we now have a good idea of what kinds of planets are out there, where they orbit, and how common the different types are. What we lack is a good sense of what that implies in terms of the conditions on the planets themselves. Kepler can tell us how big a planet is, but it doesn't know what the planet is made of. And planets in the "habitable zone" around stars could be consistent with anything from a blazing hell to a frozen rock.
The Transiting Exoplanet Survey Satellite (or TESS) was launched with the intention of helping us figure out what exoplanets are actually like. TESS is designed to identify planets orbiting bright stars relatively close to Earth, conditions that should allow follow-up observations to figure out their compositions and potentially those of their atmospheres.
Right now, there's a conference happening that's dedicated to describing some of the first discoveries made using TESS. Those discoveries include a three-planet system that seems perfectly positioned to test all of our exoplanet characterization techniques.
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The new system
The new three-planet system is called TOI-270, and it's about 75 light years from Earth. The star at the center of the system is a red dwarf, a bit less than half the size of the Sun. Despite its small size, it's brighter than most of the nearby stars we know host planets. And—critically—it's stable. That means that variations in the star's light are minimal, and they're less likely to get in the way of trying to pick up subtle changes caused by its orbiting planets.
The three planets have orbital periods of 3.4 days, 5.7 days, and 11.4 days. The ratio among these periods can be expressed as ratios of integers, a feature that's called "orbital resonance."
These resonances tend to stabilize the orbits, keeping the planets' interactions from ejecting one of them from the system or send one diving into the star. Based on the size of the planets, the trio consists of a super Earth as the innermost planet, while the two outer planets are somewhat larger, falling into the class termed sub-Neptunes.
Right now, we only have enough observations of the TOI-270 system to confirm the existence of the three planets. Orbital simulations, however, suggest that a wide range of orbital eccentricities would be stable in the system, so it will take extended observations to figure out the precise details of the orbits.
But, since the longest orbit is under 12 days, that's not so onerous. Once the orbits are figured out, the planets are close enough together to cause transit-timing variations, providing us one avenue toward getting the masses of the planets. They're also large enough and close enough to the star to drag it around a bit while they orbit, creating Doppler shifts that would allow an independent measurement of the mass.