on the Proposed Planet orbiting Proxima

Recent news has announced with breathtaking excitement that astronomers have discovered a planet orbiting Proxima Centauri!  Or, rather, the information has leaked, and hasn’t yet been officially released, and lots of disgruntled scientists (like me) are wishing things had gone a bit differently. Why? Well, it’s common these days for research to get around (and even to have its full time in the national/scientific spotlight) prior to official peer-reviewed publication thanks to press releases and preprints on ArXiV.org. This time, though, the news was leaked BEFORE the press release and before any preprint was available to be seen.

Red dwarf star CHXR 73 A and companion object (artist's concept)
Planet orbiting a red dwarf. NASA/ESA/G. Bacon (STSCI)

What does that mean? Well, for one thing, I know barely any more than you do about this discovery. There are no facts, and not even a certainty that there WILL be a press-release. There’s still a possibility that the astronomers might have re-checked their results and discovered, on further analysis, that the planet doesn’t really exist (I’ve had super-exciting results that, on further deeper checking, turned out to be not so great…). Of course, that means the final legacy of this news story will probably be conspiratorial “are they hiding something?” messages.


Why is the possibility of such a planet so exciting? Well, as you may know from school, or TV specials, or science news, or the game Sid Meyer’s Alpha Centauri, the alpha Centauri star system is the closest star system to the Solar System, and Proxima Centauri (=alpha Centauri C) is the closest member OF that triple star system. It’s 4.2 light years away, which is a very long way, but there is genuinely nothing closer*.

To put it into perspective, earlier this year, the Breakthrough Starshot initiative was launched suggesting that it was within the realm of physical possibility to send postage-stamp-sized space probes to other stars at 20% the speed of light. Warp 7 it ain’t, but that velocity would still make it feasible to reach a few star systems within a human lifetime: Proxima Centauri in 21 years, Barnard’s Star in 30 years, Sirius in 40 years, Procyon in 55 years… In total, there are about 22 star systems (going off this list, plus the two brown dwarf systems Kevin Luhman has found) that could be reached in under 60 years at that speed, but regardless of how fast we can go… Proxima Centauri will always be the closest (for the next 33,000 years, anyway). A planet worth visiting in orbit of Proxima Centauri would be the absolute best possible case for exploring the Galaxy.

So what do we know? The discovery was supposedly made by the European Southern Observatory, and it’s supposedly of an Earthlike planet. That could mean roughly the same mass, or within the habitable zone of Proxima Centauri (where the planet is just warm enough for water to exist as a liquid on the surface), or both.

The European Southern Observatory does not have, as far as I know, any dedicated transiting planet surveys. That said, Proxima Centauri is a special case, so it’s possible they might have had a special campaign to see if there was anything there. But I’m going to assume this discovery was made using radial velocities.

That means the planet was detected by its wobble – the reflex motion of Proxima Centauri towards and away from us as it’s pulled gravitationally by the planet orbiting it. If the planet is pulling it directly toward and away from us, they’re seeing the maximum effect and the minimum size of the planet (and should see it passing in front of the star, and I’d be shocked if no one noticed that before). If we’re looking down on the orbit, that means we’re only seeing part of the pull the planet has on Proxima, and the planet could be much, much bigger. In short: they know the minimum mass of the planet, and the orbital period of the planet. And that’s basically it. Nothing about the radius, nothing about the atmosphere, nothing about whether it’s actually rocky, or actually has water on its surface, or life.

On the plus side: Proxima Centauri is a very small, very dim, very low-mass star, so the effect of an Earth-mass planet on the star would be much bigger and easier to detect than around sunlike stars like alpha Centauri A and B.

On the minus side, Proxima is deep orange/red and prone to massive stellar flares, and very cold as stars go. Proxima Centauri’s habitable zone/liquid water zone is VERY close to the star. That means a short year – 5-20 days long. And the planet would almost certainly be tidally locked to Proxima, such that one side of the planet would always face the star, and the star would hang more or less motionless in the sky. (alpha Centauri A and B would merely be bright lights in the sky; they’re too far away from Proxima to seriously affect the planet).

Needless to say, “habitable” to astronomers doesn’t really mean all that similar to Earth. We’re kind of starved for comparisons at the moment, so we take what we can optimistically get.


Another potential issue:  I mentioned that Proxima Centauri is prone to massive stellar flares. It’s also full of sunspots, and because it’s so cold (compared to other stars, anyway) it has a very complicated spectrum. To make such detailed measurements of a messy, noisy, complicated star is very difficult, and the eventual paper is going to have to demonstrate a lot of careful work before astronomers will be convinced that they’ve really done it.

We’ve been down this road once before with the alpha Centauri system. In 2012, a planet was announced orbiting alpha Centauri B. It was Earth-radius, but with a 3 day orbit. Because alpha Centauri B was a sunlike star, the planet would have been seas-of-magma hot. But hey, it was a planet only 4.3 light years away!

Except it wasn’t. The amazingly tiny signal (the star’s moti0n was 51 cm/s, the speed of your average New York City pedestrian) that they thought was a planet turned out to just be random noise.

I worry that this Proxima planet will also turn out to be a mistake, particularly because (if I take my scientist hat off for a second) I really really want it to be real. But for that, we’ll need to actually need to have some facts and figures to judge it by.



* There is, admittedly, still a small chance of finding a dim brown dwarf closer than Proxima.

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