Astronomers have found an exoplanet orbiting the closest single star to the Sun!

This is a kinda big deal. Except it’s also a small one. And maybe a lot of small ones.

We’ve already found planets orbiting stars in the Alpha Centauri system, the closest to Earth. But that’s a trinary star, three stars orbiting each other.

The next closest to us is Barnard’s star, a dim bulb of a star just under 6 light-years from us. It’s an M4 red dwarf, about one-sixth the Sun’s mass, half the temperature, and one-fifth its diameter, shining at only 0.0004 of the Sun’s luminosity (in visible light, that is, the kind our eyes see; it’s brighter in infrared and the star’s total luminosity is about 0.003 solar). Replace the Sun in our solar system with Barnard’s Star and it would only look about 10 times brighter than the full Moon! You’d have to squint a bit, but it would shine so feebly it would look like twilight even at high noon (darker, really since the star is so red; the sky would be very dark as well).

Just a few years ago a planet was reported to orbit the star, but not long after that was shown to be a false alarm. Barnard’s Star, like so many red dwarfs, is magnetically active, much more so than the Sun. That means it has lots of starspots (like sunspots but on another star) and stellar flares, and these can throw off the observations. 

The new observations, though, take this into account [link to journal paper]. They find pretty convincing evidence that Barnard’s Star does indeed have an exoplanet, and maybe more than one!

They used the radial velocity (also sometimes called the reflex velocity) method. Ironically, I described this in detail on The Old Blog™ in an article about finding that spurious planet around Barnard’s Star! Heh. Anyway, the idea is that a planet orbits a star, of course, but the planet has gravity as well, so in reality the star and planet orbit around their center of mass, technically called the barycenter. This motion is in principle detectable; although the circle they each make is incredibly tiny and almost never directly seen, as the star moves around the barycenter its light is alternately blue and red shifted by the Doppler effect.

That shift is pretty small. It’s bigger if the planet is big and/or the star is small, and in this case Barnard’s Star is a stellar lightweight, so that helps. But it’s also faint, so getting good spectra is difficult. Worse, starspots on its surface can mimic this shift as the star rotates, messing up the observations.

In these new observations, astronomers used the huge Very Large Telescope, a set of four 8.2-meter behemoths in Chile. They took 157 observations of Barnard’s Star over the course of five years, from 2019 to 2023. The observations were spaced out in time in a way to look for the change in the Doppler shift of the star for planets that had orbits that took less than 50 days. Why that length?

Red dwarf stars are dim, and cool. A planet orbiting one as far as Earth is from the Sun would freeze solid. To be warmer and Earth-like, a planet would have to be much closer to the star, with an orbit shorter than 50 days. The astronomers were therefore specifically looking for planets in the star’s habitable zone.

After a lot of data processing, including removing effects of the star’s rotation and activity, they found pretty good evidence of a periodic Doppler shift that takes about 3.15 Earth days to complete. That indicates a planet with that orbital period. They can also measure the planet’s mass — again, the more massive the planet the larger the shift — and they find it has to be at least 0.37 Earth masses. That’s three times the mass of Mars, or a bit less than half of Venus. 

Why “at least”? Because we don’t know the orbital tilt we see the planet at. If we see the planet’s orbit exactly edge-on, then the planet has the mass determined. But it might be tilted, so we see it at an angle, or even nearly face-on. That introduces a trigonometric sine into the math; the closer the angle to face-on the higher the mass has to be to generate the Doppler shift measured. So the planet might be orbiting nearly face-on, and the planet more like Earth in mass.

At that distance from the host star, they find the planet has a temperature of roughly 130°C, or 260°F. That’s too hot for life, but that assumes the planet is fairly dark and is good at absorbing light from the star. If it’s cloudy, say, or just whiter, it could have a lower temperature. On the other hand, if it has an atmosphere with a lot of greenhouse gases like carbon dioxide, it could be a lot hotter, like Venus. There’s no way to tell for now.

Either way, it’s unlikely to be like Earth. More like a hot Mars. But it seem to be real, and if so will be called Barnard’s Star b (or possibly GJ 699b, since GJ 699 is the star’s more official name).

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