25 August 2016

Another "Earthlike planet"

The recently-discovered planet "Proxima b", orbiting the star Proxima Centauri, is being hailed on news websites as a possible "Earthlike planet" -- a term which crops up with some regularity in such reports.  Planets beyond our solar system, in and of themselves, are no longer newsworthy -- we've discovered well over three thousand of them -- but "Earthlike" suggests a place with plants, animals, "normal" days and nights, and of course the comfortable temperature and oxygen-rich atmosphere that almost all planets in science-fiction movies miraculously have.  There might even be Wookies or Klingons or Greys living there.

Proxima b is actually interesting from one viewpoint -- it's the nearest extrasolar planet ever discovered.  Its sun Proxima Centauri is part of the Alpha Centauri system, which is our own solar system's nearest interstellar neighbor.  The two main stars in the system are Alpha Centauri A and B; they are fairly similar to our own Sun and orbit each other at a distance ranging from about one to three billion miles (one billion miles is about the distance from here to Saturn).  Proxima Centauri is a much smaller and fainter "red dwarf" star orbiting the main pair at a distance of about one and a half trillion miles, which is one-quarter of a light year.  It is currently between the main pair and us, making it the closest star to our Sun.  The whole Alpha Centauri system is about 25 trillion miles (over four light years) from us.

(For context, 25 trillion miles is about seven thousand times the distance from Earth to Pluto, and it took the New Horizons space probe almost ten years to get to Pluto.  "Near" is a relative term.)

Proxima b is similar in mass to Earth, though a bit larger, and it orbits within the "habitable zone" of Proxima Centauri -- the zone within which, if certain other conditions are met, the range of temperatures would be right for liquid water to exist.  It is these features that prompt the designation "Earthlike".

We don't actually know much more than this about Proxima b yet, but a few things can be deduced.  Remember, Proxima Centauri is a much smaller and fainter star than our Sun, so its habitable zone is much closer in.  In fact, whereas Earth is 92 million miles from our Sun, Proxima b orbits less than five million miles from Proxima Centauri.

That is within the "habitable zone", but with such a small orbit, the "year' is only eleven Earth days long.  More importantly, being so close in, the planet is almost certainly "tidally locked", meaning it keeps one side permanently facing Proxima Centauri and the other side permanently facing away, as the Moon is to the Earth.  Half the world has eternal day, and half eternal night.  Under these conditions the oceans, if they ever existed, must have long ago frozen solid on the night side and boiled away on the day side (ultimately ending up as precipitated snow on the night side), while the atmosphere, if there ever was one, would also have frozen out on the night side.  It could be that more reasonable temperatures persist in a narrow ribbon of territory between the two hemispheres, but since the planet is probably airless, liquid water still could not exist there.

Numerous other factors can affect conditions on a planet -- Venus, for example, is similar in mass to Earth and only 25% closer to the Sun, but with its 900-degree temperature, its oxygenless air, and its atmospheric pressure about ninety times that of Earth, it is not remotely habitable to humans.

Earthlike planets may exist, but everything we know so far suggests that they are not common, and Proxima b certainly is not one.

5 Comments:

Blogger Paul Wartenberg said...

In other news, NASA is quickly working on a long-range prison ship that they can launch Trump for Promixa B and hope for the best.

25 August, 2016 11:07  
Blogger Infidel753 said...

Maybe the thing on his head is from there.

25 August, 2016 16:51  
Blogger Ranch Chimp said...

Yeah, actually I read a little on this the other day ... you're right, everything today is earth- like, eh?, heh, heh, heh, heh, heh {:-) But I wondered when I first read it about that half of the planet day or night thing. BUT ... the positive to this, is at least we learned a Hell of alot more over the years ... and you have alwayz said that earth was unique I remember (you're the one that told me about "Rare Earth" book) ... being I guess what you mean, is that when folks advertise "earth- like" ... it's not the same as the real thing being "life- like". I think there is a Hell of alot more out there to discover, and it would not surprise me a bit, if they ever found a planet with forms of life like here.

26 August, 2016 04:14  
Anonymous Zosimus the Heathen said...

That was an interesting read. While I've long known of the Alpha Centauri system, and recall hearing that it contains multiple stars (I've heard that multiple-star systems are actually the norm in the cosmos rather than anomalies), this is the first time I've actually read an explicit description of what stars make up the system, and their spatial relationships with one another. Surely it's a testament to the vastness and emptiness of space that a star three "light months" from a couple of others can be considered part of the same system that those other two stars inhabit.

Sadly, as you say, it's unlikely that Proxima b would be a suitable habitat for life, at least any form of life we'd be remotely familiar with. Just another reminder, I suppose, of how important it is we look after our own planet - there very likely might not be another like it in the universe!

26 August, 2016 11:07  
Blogger Infidel753 said...

Ranch: If we ever did find another planet with complex life, it would be one of the most exciting scientific finds ever. I think they're incredibly rare, though, if they exist at all.

Zosimus: Thanks! Yes, multiple-star systems are commoner than singleton stars like our Sun. This is another reason why habitable planets are probably rare -- a multi-star system makes stable orbits difficult. Alpha Centauri is actually an exception. The two main stars are just far enough apart that either one of them could have an "Earth" in a stable orbit around it. So far we've found no sign that any such world actually exists, though.

By the way, the picture at the top of this post is a joke. It's not a picture of Proxima b (we'd have no way of taking such a picture anyway), nor of any other planet. It's the underside of an old frying pan. The eye tends to see what it wants to see.

26 August, 2016 18:12  

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