on the Proxima Centauri planet

According to ESO’s latest press release, there is indeed a small slightly-larger-than-Earth-mass planet (1.3 earth masses) in orbit of the closest star to the solar system, Proxima Centauri. It’s even close enough to the star that liquid water could exist on the surface. That’s mighty close indeed – the year is only 11 days long! It takes less time for this planet to orbit Proxima Centauri than it does for the Moon to orbit the Earth (it is in a larger orbit than the Moon’s around the Earth, though).

Artist's impression of the planet orbiting Proxima Centauri
Artist’s Impression of the planet orbiting Proxima Centauri (with alpha Centauri A and B visible just above and to the right of Proxima). Credit: ESO/M. Kornmesser

So what comes next?

It’s not “pack your bags and head to Cape Canaveral” time. Even assuming the planet is real – and the paper has a steep job of proving that they really did extract a signal of 1.3 meters per second (a speed would let you finish a 5k race in an hour) around a messy, noisy, dim M dwarf star, when the 0.5 meters per second around the comparatively well-behaved alpha Centauri B turned out to not be real – there are a lot of things nobody knows yet. And “habitable” does not mean what you probably think it does.

Put simply, the “habitable planet” designation used by astronomers is not the same as the “M Class Planet*” designation used on Star Trek. When Star Trek says the planet is “Class M”, that means they can beam down a couple of redshirts in standard duty uniforms (no space suits), and expect them to survive at least until the monster-of-the-week finds them. Sure, some planets are unusually cold, unusually hot, unusually wet, unusually dry, incredibly stormy… but they’re all basically you-could-live-here places.

When astronomers say “habitable planet”, we don’t actually mean that the planet is habitable by life (as we know it) and inhabited by little green furry things with ravenous appetites for Ensign Ricky. We mean that the planet is close enough to the star that IF it has an earth-like atmosphere and IF it has water, that water could exist as a liquid. That’s it. There’s no guarantee any of those things are actually true. Remember: the Moon is within the Solar System’s habitable zone, as is Mars according to some definitions – there is limited liquid water on its surface, after all. If we set more restrictive parameters, we quickly end up with just one example: Earth. And that’s not entirely fair either because we just don’t have the technological capability to say if planets in other solar systems are worth setting up summer homes on. To put it another way: all colonizable planets will be “habitable zone” planets… but not all habitable zone planets will be colonizable. And we have no way of knowing which is which right now.

So, as a mere habitable zone planet, we don’t actually know if Proxima b is made of rock, gas, or styrofoam (although planet formation theories heavily point toward few-earth-mass objects like Proxima b being rocky). We don’t know if it has an atmosphere. Even if it does have an atmosphere, we don’t know if it’s Oxygen/Nitrogen or Carbon Dioxide/Methane or Fluorine/Neon. And once we figure that out, does it actually HAVE water on the surface? Answering that question will require spectroscopic measurements that block out Proxima Centauri so we can see the atmosphere of Proxima b. This will be particularly tricky given how close Proxima b is to Proxima – it’s at most 7 million kilometers away, which at a distance of 4.2 light years is an angle of 35 milliarcseconds (the brand-new Gemini Planet Imager’s capability is 200 milliarcseconds at its narrowest) – but you can bet astronomers will try, and even build special-purpose equipment to do it.

proxima_2010
The proper motion of Proxima Centauri (Rigil Kentaurus C) as seen on CTIOPI astrometric images from 2000-2010. Each pixel in the unresized image is about 400 milliarcseconds across, which translates to 75 million kilometers at the distance of Proxima Centauri. Needless to say, the planet is not visible.

After that: Is it time to pack your bags and head to Cape Canaveral? Unfortunately not.

As I said last week, this planet is over 20 years journey away even with our most pie-in-the-sky projections of achievable velocity, and even that is for a probe with the mass of a postage stamp. It’s not in our technical wheelhouse to even try such a thing right now. Pluto’s distance from the Sun is 39 AU (39 times farther from the Earth than the Sun); Proxima Centauri is 268,000 AU away from the Sun. But when we DO start sending out interstellar space probes, we will definitely be sending them to Proxima b.

At least Star Trek was right about one thing: there really are planets everywhere.

* Star Trek’s “M Class Planet” moniker also has absolutely nothing to do with the spectral class M for stars. M-type stars (like Proxima Centauri) are between 8% and 60% the mass of the Sun, and they’re between 100 and 1 million times fainter than the Sun. M dwarfs have surface (well, the highest point at which they’re opaque) temperatures ranging from 2,200 to 3,800 degrees Kelvin (Water boils at 373 Kelvin; lava of the kind you can find in Hawai’i is around 1,400 Kelvin; the Sun is around 5700 Kelvin).

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