Nearest Star Has a Potentially Habitable Planet!

The “Pale Red Dot” campaign at ESO has just announced a discovery that is truly extraordinary!  This would have been my dream discovery as an astronomer! I feel really lucky (and a little giddy) to have gone from simply wondering if there really were planets around other stars to finding this in my lifetime! I hope we will find out more about it in the years to come, and that someday our descendants will get to see it up close.

Proxima Centauri

The Pale Red Dot campaign was set up to search for possible planets around the nearest star, Proxima Centauri (a.k.a. Alpha Centauri C). (Image Credit: ESO / CC By 4.0).

They report three rather stunning things which, together, make for a highly coincidental and lucky find for us:
  1. This is the nearest star to our solar system, meaning the easiest for us to reach with an interstellar spaceprobe. It could be done with technology that would take only a decade or so to develop in about a 50-100 year flight. Actually, it’s a perfect target for something like the “100 Year Starship” project, currently led by Mae Jemison‘s Foundation which was set up to explore the problem of maintaining a support organization for such long-term projects and develop the necessary technologies.
  2. The planet is about 1.3 Earth masses in size (actually it could be a bit more, because this is a radial velocity measurement without a confirmed inclination angle, but it’s unlikely to be more than twice that mass). Based on our understanding of planetary science, that almost certainly makes it a terrestrial planet, and one which would easily be able to hold onto a thick atmosphere. It might also have a significant magnetosphere, though that’s a much more complicated issue to work out (a slow rotation rate might argue against that).
  3. The planet lies within the habitability zone, meaning that liquid water could exist in significant quantities on its surface — it could have oceans, for example.
Proxima Centauri is a very small star — an M-class red dwarf. So it’s really more like the size of Jupiter than the Sun (although quite a bit heavier), and it’s dim and reddish (although not as deep-red as sometimes depicted in illustrations — they often exaggerate the color). The habitability zone of such a star is very small — this planet orbits much closer than Mercury around our Sun, with a period of only 11.2 days.
Perhaps surprisingly, it’s actually on the cold side of the band.
Because the orbit is small compared to the diameters of both the star and the planet, the planet is likely to have become tidally-locked (like our Moon is around the Earth, so that the face of the Moon we see does not change). A second possibility is that it has settled into a 3:2 resonance, the way Mercury rotates in our solar system.
Bear in mind that these are all “model-based projections”. All we really know is orbit and mass information. But we know the properties of the star and we can extrapolate from what we know about terrestrial planets in our solar system (meaning Mercury, Venus, Earth, and Mars). Moons may be nearly as relevant, since such small orbits resemble the orbits of moons around the large gas-giant planets in their dimensions.
The Earth is, however, the largest terrestrial planet we have been able to study up close.
If we make some assumptions — namely that the planet has an atmosphere just like Earth’s and the surface is an ocean (not a bad approximation for the Earth, which is about 70% ocean), then we can project a map of what the surface temperature would be like. The researchers did this with an existing climate model developed for Earth, running two separate simulations — one with the 1:1 rotation/orbit assumption (“tidally locked” like the Moon) and the 3:2 rotation/orbit assumption (“3:2 resonance” like Mercury).
Under either of these assumptions, more than half of the planet would persist at temperatures between -15 C and +30 C, well within the range of active ecosystems on Earth. Unlike Earth, the temperature would not change much over time. The weather would be very stable, with conditions more dependent on geography than time — life forms might migrate inwards or outwards to find habitats more suited for them (or to escape predators that couldn’t stand the conditions).
About the only “weather” I would expect would be due to flare activity from Proxima, which would probably cause some atmospheric disturbances by extra heating and X-radiation.
The tidally-locked case is actually the more habitable case, with the “hot pole” hovering at +30 C, and the freeze-line lying most of the way to the terminator. The coldest point in this model is around -75 C, which is pretty cold, but not colder than the coldest record temperatures on Earth (parts of Antarctica can get below -80 C occasionally).
Video turn-around of the tidally-locked model:

(http://www.eso.org/public/videos/eso1629g/)

In the 3:2 model, the warm region is just above freezing, but covers a large equatorial band.

Video turn-around of the 3:2 model:

(http://www.eso.org/public/videos/eso1629h/)

Of course, the atmosphere is likely different than Earth’s, and that expands the range of possibilities considerably. For example, a thicker atmosphere might both warm the planet by additional greenhouse warming and distribute the heat more evenly. A surface not covered by oceans would, on the other hand, probably show more temperature extremes.

It is easy to imagine this place having evolved life forms. In fact, it’s kind of hard not to. 😉

From an exo-biological perspective, the worst property of this planet is that it probably gets bathed in quite a bit of X-ray emission from Proxima, which is an active flare star. Any life to evolve in such an environment, would have to have very high natural resistance to them.

Of course, there are all kinds of caveats. We know absolutely zip about this planet from direct observation. We only have “M sin i” (meaning the minimum mass it can have) and orbital parameters (except for i — the inclination). This is the usual case with radial velocity discoveries of extrasolar planets, unless you just happen to be lucky enough to catch a planet that is aligned so that it passes in front of the star (in which case, i is pretty nearly 90 degrees).  In this case we know it doesn’t, so i is a bit less than 90 degrees. It could be a more massive planet seen practically pole-on, but that’s just very unlikely geometrically. There are also some indirect indicators that favor a high inclination — a second detected planet apparently does transit the star, which puts its inclination near 90 degrees. We think planets in a planetary system will tend to lie in the same plane as they do in our solar system, so it’s likely that this planet is not anywhere near pole-on.We only presume from the mass that the planet is terrestrial. We’ve never seen a gas-giant (or “ice-giant” as some people call Uranus and Neptune) which is this light, but then we haven’t seen many planets directly. Direct observations of extrasolar planets are extraordinarily hard, not only because they are dim, but because they are right next to the much brighter light source of the star. It may be quite some time before we hear about any direct observations, although for such observations, the distance is really important, and this is the absolute closest star system to our own (On the other hand, the orbit is also very close, which makes it harder).

ESO has also published an interview with the Pale Red Dot project scientists, Pedro Amado, Guillem Anglada-Escudé and Ansgar Reiners:

 

 

THIS is a reason to build a starship! I know I want a closer look!

 

 

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About Terry Hancock

Terry Hancock is the producer and director of "Lunatics" ( http://lunatics.tv ). He is also a regular columnist for Free Software Magazine ( http://www.freesoftwaremagazine.com/articles_by/5 ), and a lifelong advocate for space, science, and technology. More at http://terryhancock.narya.net
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