on the Closest “Habitable” Exoplanet

Exoplanet orbiting a small M dwarf (JPL/Caltech)

This week, an planet orbiting Wolf 1061 (aka GJ 628, aka LHS 419, aka HIP 80824, aka a bazillion other names because astronomers are like that) was announced. The planet Wolf 1061c is notable for three important reasons.

  • It’s potentially only 4.25 Earth masses, and is therefore likely to be a rocky world.
  • It’s found within the habitable zone of Wolf 1061
  • Wolf 1061 is only 4.26 (+/- .027) pc away, or somewhere between 13.8 and 14 light years away (which makes it the 30th closest star system)

This isn’t another contender for most earthlike planet. Rather, this is a contender for the title of “closest Earthlike planet”, beating out Gliese 667c, which is 4.5 times the mass of the Earth but 22 light years away (and not even in the top 100 closest systems).

Under the assumption that it’s a ball of silicate rock, the team at the University of South Wales estimate the radius at 1.64 Earth radii. (a radius which, according to my previous post, may mean it’s more likely to be a mini-Neptune, but let’s go with it…*) Increasing the radius (the force of gravity is F=GMm/(r^2)) means that quadrupling the mass (M) wouldn’t lead to quadrupling the gravitational force (F) on a hypothetical alien or colonist on the surface… but the surface gravity WOULD be higher if the star is less than 2 Earth radii. With 1.64 earth radii, we’re talking about  48% stronger gravity on the surface. Assuming there is a surface, of course.

The Wolf 1061 itself is an M3V star, about 1/4 the mass of the Sun, 120x dimmer, and most of its energy output is in infrared light we can’t see… That means that in order to be the right temperature for liquid water (which, mind you, is no guarantee it HAS liquid water), this planet has to be CLOSE to the star. And it is: it’s got a 17.625-day year, and is only 0.08 AU from the star (five times closer than Mercury is to the Sun, but remember, Wolf 1061 is a very dim star). Wolf 1061 would therefore be HUGE on the sky: four times the diameter of the Sun as seen from Earth. And the star would only (and always) be seen from one hemisphere of the planet, because the planet is to be tidally locked to Wolf 1061 the same way that only one side of the moon is visible from the Earth. So, not exactly Earthlike, but there could be water on the lit half.

This discovery was made with publicly available data from the HARPS instrument, which means someone else looked at this data before, and found nothing. If you take a look at the velocity curves in the paper (page 20, figure 3), there does appear to be something there… but it really needs to be shown that this isn’t some kind of pulsation or dimming on the star itself (small stars are known for that sort of thing). The authors are pretty sure they’ve done this, but I’m sure that’s where most of the critical evaluations are going to be directed.

Of course, at a distance of 13.9 light years, this closest star is STILL well out of our reach. Heck, even the now-debunked planet around alpha Centauri B (the closest star system, 4.3 light years away) would be out of reach right now. It is, in many ways, a frustrating time to be alive.

*Maybe it has a moon. With Ewoks.


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