Bad astronomy | The red dwarf LP 890-9 has two super-Earths, one in the habitable zone
Very cool exoplanet news, literally: Two super-Earth planets have been found around a red dwarf star, and one of them is the right distance from its star to – maybe – be reasonably temperate.
The star is called LP 890-9 — the LP stands for Luyten/Palomar; Using the Large Telescope, astronomer Willem Luyten mapped and cataloged thousands of nearby stars. LP 890-9 is a very small star, a red dwarf with only 1/9e of the mass of the Sun and 1/6e its diameter. Its temperature is only 2,580°C (4,670°F), much colder than that of the Sun, and it emits an incredibly small amount of 0.1% light compared to the Sun. At 100 light years, relatively close, it is so faint that you need a large telescope to see it [link to paper].
The first planet, LP 890-9b, was discovered by TESS, the Transiting Exoplanet Survey Satellite, which scans the entire sky looking at thousands of stars. If we happen to see a star with a planet orbiting sideways from our vantage point, once per orbit it blocks out some of the starlight, an event called a transit. Knowing the size of the star, measuring the amount of light blocked gives the size of the planet.
In this case, TESS saw a dip in light, but the uncertainty was high, so astronomers followed up with SPECULOOS telescopes. That means Ssearch for living quarters Planets THISlips ULtra-cOOI Stars, a project designed to examine weak red dwarfs for planets. Observations from SPECULOOS nailed the transit, seeing a 1% dip in starlight, indicating the planet is 1.32 times the diameter of Earth. This makes it a super-Earth, a planet larger than ours but still much smaller than Neptune, the next largest planet in our solar system.
It orbits its star once every 2.73 days, which means it is within 3 million kilometers of the star. Is it close. Still, the star is a dim light bulb, so the planet only gets about four times as much energy from the star as we get from the Sun, making it probably around 120°C (250°F). ). Not habitable from a distance.
This is where it gets fun. SPECULOOS observations have indicated that there is a second planet orbiting LP 890-9. Called LP 890-9c, it is about 1.37 times the diameter of Earth, or about 17,500 km wide: another super-Earth.
It is also in a wider orbit, taking 8.46 days to orbit the star at a distance of around 6 million kilometers. Still close, but again the star is cold, so the planet’s temperature may be 0°C (30°F) warmer. Yes, it may sound cold, but Earth’s temperature without greenhouse gases would be even colder! So if this planet is rocky and has a hint of carbon dioxide in its atmosphere, it might actually be temperate.
The problem is that we don’t know what the masses of either planet are. It’s needed to find their densities, and it gives us a good idea of what they’re made of. A gas giant has an average density close to that of water, while a rocky/metallic planet like Earth has a density more than 5 times greater. A lower density could also indicate that it is an aquatic world.
The mass of an exoplanet can be found using the radial velocity method, but it’s difficult to use on really faint stars. Perhaps larger telescopes could be used to gather enough light to make these observations, but it takes months of observations to accumulate enough data. So it may take some time.
But it should be done. LP 890-9c is, on paper, perhaps the second-best candidate to resemble Earth, after TRAPPIST-1e, an Earth-sized planet orbiting the tiny red dwarf star TRAPPIST-1. This star is the coolest known to have planets, and LP 890-9 is the second coolest.
This makes LP 890-9c an excellent target for JWST observations, to see if it has an atmosphere and perhaps work out what’s in it. This has already been done for TRAPPIST-1e, although the results have not yet been published. Yet, in our search for other Earths, they are excellent starting points.
I love it when you find planets around red dwarfs. They are by far the most common type of star in the universe, they tend to make smaller, Earth-sized planets, and they tend to make a lot of them. Our own world may be an exception in its class, orbiting a white G dwarf and not a red M dwarf.
The Universe may be filled with rocky worlds orbiting small, faint stars. And we are only just beginning to find them.
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