Image courtesy of Luciano Mendez, a hypothetical view of moons around a giant planet in the habitable zone.
Exoplanets. And if we’ve already found over 700 with just a few years of looking then how many billions really exist? It’s a crowded universe.
Exoplanets are planets that orbit other stars. Most we have found so far have been what are called ‘hot Jupiters.’ These are very large gas planets that orbit close to their star and very fast – some are closer than Mercury. They whizz around their stars in a matter of days or even hours and would be completely hostile to us. It sort of gives the impression that Earth is a lonely exception, rocky and in the right zone for liquid water.
But this has more to do with the way planets are detected than how common they really are. Our telescopes aren’t good enough to observe exoplanets directly, we have to detect them by how they affect their parent star. It’s actually amazing that our computers can detect changes in something so far away we can barely see them ourselves, caused by something we can’t see even with our best telescopes.
Most exoplanets are found by studying the tiny changes they cause in their star’s position and movement. Others are found through transits, when they cross between us and their parent star. When they go in front of the star they are enough to dim its light in a regular and predictable way, showing that there is something orbiting. Of course it’s a lot more complicated than that, there are all sorts of checks to see that it isn’t something to do with the star or caused by other interactions, but that’s the basics for kids’ science.
Using these techniques it is much easier to pick up planets that are big, because they make a bigger change, and fast so we can see several orbits. It would literally be impossible to pick up something like Neptune which takes nearly 165 years to orbit! So of course we have found lots of hot Jupiters, they are big and quick and easy to spot. However, things are getting exciting with even better telescopes.
The Kepler telescope has just detected two planets orbiting the star Kepler 20 that are earth size. This is brand new, just published in December 2011. At the moment it’s mainly a lot of maths to work out if they’re really there or if the changes are caused by something else, but chances are good.
There appear to be two planets, one slightly smaller than Venus orbiting in 6 days(!) and the other almost exactly the same size as Earth, orbiting in 19 days. And it seems possible that the second would have a thick water atmosphere. Any guesses about their composition are just that, based on their size, orbit and what we know about solar system formation. But if they formed further out in the ice zone then spiralled in, the second could have kept its water in a protective layer. To find something so earth-like with so many constraints on what we can detect is nothing short of incredible. Maybe the sci-fi authors are right and there are habitable planets everywhere.
So Where’re the Aliens?
This is the paradox – where is everyone? We’ve found all these planets so quickly, which suggests there are a lot more. And given probability and the huge numbers involved, space should be crowded with ships and radio waves zipping all over the place. This is called the Fermi Paradox, and in fact there’s some maths which suggests the galaxy could have been filled in 5-50 million years, which would have been way before we came on the scene. But there is no evidence of any of them.
It’s one of the big, or rather enormous, puzzles of cosmology. Maybe we don’t see the ships because they’re ducking into other dimensions to defeat the light speed limits. Maybe they’ve given up something as primitive as radio (but the old signals should still be bouncing around). Or ultra-depressingly, maybe there is no way around the light speed limit and no cryopreservation and we’re all stuck in our own solar systems because it would take too long to go anywhere else. We have no idea where they are and why they aren’t here.
What Are They Like?
Being a biologist, I like to pose a slightly different question – maybe the reason they’re not here is because they’re not interested, and maybe we can’t find them because we’re looking for the wrong thing.
Don’t get me wrong, I grew up on Star Wars and I love Star Trek. But the chance of aliens that look like us, or even giant teddy bears? Zero, zilch, zip, nada. You can make quite a good argument that intelligence is common because it’s highly adaptive, but bilaterally symmetrical bipeds with four limbs and a head at the top? Nope. We’re not even a very good design, an engineer would come up with much better.
So what is out there? I don’t know, there are people who study this whereas I’m just an armchair blogger. But think of a hot Jupiter – how fast would they live with 6 day ‘years’? How would they deal with the incredible radiation? How far down is there anything solid, or do they swim through the atmosphere? How do they get their energy? Would a life-form not based on water even have a single ‘body’ or could it be distributed somehow?
You get the picture – if it came here, would we even recognise it was alive? And why would they come here when we don’t have that sort of real-estate?
On the plus side, the future might be a lot more peaceful than sci-fi suggests if we aren’t competing with alien species. But it might also be a lot lonelier.
This interactive includes art, video and stats on seven of the known exoplanets and is well worth a look.
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