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SYFY WIRE Bad Astronomy

Exoplanet news: TESS finds a wee world orbiting a wee nearby star

By Phil Plait
Artwork comparing the sizes of Mars and Earth to the three planets orbiting the red dwarf L 98-59. We don’t know what the planets actually look like, but we do know their sizes and masses. Credit: NASA Goddard Space Flight Center

I'm starting to get the idea that maybe the Universe likes making interesting planets.

After all, we have 8 of our own orbiting the Sun… but with more than 4,000 exoplanets under our collective belt now, it turns out a lot of them are really cool.

The Transiting Exoplanet Survey Satellite, or TESS, is finishing up its first year of observations from Earth orbit. It has already found over 750 "objects of interest," that is, exoplanet candidates, with 20 having been confirmed so far. A lot more are coming: TESS is observing something like 200,000 nearby stars, and is expected to find tens of thousands of planets!

One of these stars is L 98-59. It's a red dwarf, a tiny, cool, dim bulb of a star just 34.5 light years from Earth. It has about a third of the Sun’s mass and size, and only about 1% of the Sun's brightness. Even at its relatively close distance you'd need (big) binoculars to spot it. TESS found three planets orbiting it (so far), and all of them are roughly Earth-sized. This isn't as surprising at first blush as you might think, because we know that these M-class stars tend to make smaller planets (like the TRAPPIST-1 system).

Two of the planets (called L 98-59 c and d) are super-Earths, with diameters 1.35 and 1.6 times Earth's. That's still small enough that they may be rocky worlds like our own planet. But it's the third planet (L 98-59 b) that's really fun: It's smaller than Earth, about 0.8 times Earth's diameter!

Finding such planets is tricky, because the transit method — looking for dips in the amount of starlight we see as a planet passes directly in front of the star as it orbits — favors bigger planets. Planets that are the size of Earth or smaller* are tougher to detect. This is, so far, the smallest planet TESS has found (though smaller ones have been found previously, like Kepler-37b, which is smaller then Mercury!).

All three planets orbit the star very closely, and despite the star's diminutive nature, are blasted by heat. Planet b orbits once every 2.3 days, while the bigger planets c and d have orbits that are 3.7 and 7.5 days.  When you’re that close to a star, even an M dwarf, it gets warm.

The mass of the smallest planet isn't known, but another team of astronomers observed the system using HARPS (using what’s called the reflex velocity method) determined that the two outer planets have masses of about 2.5 and 2.3 times Earth's. That means they are likely rocky worlds with at least a passingly similar composition to Earth (meaning metal cores and rocky upper layers, as opposed to being more like gas giants).

Artwork comparing the sizes of Mars and Earth to the three planets orbiting the red dwarf L 98-59. We don’t know what the planets actually look like, but we do know their sizes and masses. Credit: NASA Goddard Space Flight Center

They were able to get an upper limit for the mass of the smaller world, and found it to be no more than 0.98 times the mass of Earth. That means it, too, is a rocky world. Remember though, all three are hot hot hot, so they can’t support life as we know it.

The good news here is that these planets are excellent candidates for follow-up observations to look for atmospheres. Atoms and molecules in atmospheres absorb light at specific wavelengths (colors), so we can measure the light from the planet during a transit and compare it to the light from just the star when the planet is not transiting. Looking for the wavelengths where light is dimmed can then reveal if an atmosphere exists, as well as its composition.

If this can be done, it will be very interesting to see how planets the same size as Earth evolved differently due to so much more heat from their star. Venus is the only example we have here, and though it doesn’t get that much more light than we do it's very different from Earth. The more examples of such things we have, the better we can understand our own world and the processes shaping it.

I'm eager to see what TESS has in store for us. Unlike Kepler, which stared at one spot in the sky looking at stars of all different distances form Earth, TESS is sweeping the sky looking at the brightest stars, which in general are closest to us. A lot of these stars are familiar to not just astronomers but also sci-fi fans… after all, a planet was found orbiting 40 Eridani A, which is canonically the home of Spock from Star Trek. I can't help it; I'm a scientist and also a nerd, and hearing about planets around stars I know gives me two big reasons to be happy.

*My friend and planetary science-communicator Emily Lakdawalla thinks we should maybe start using the term "Venus-sized" to prevent people from thinking these planets are Earth-like. I find it hard to argue against her point.