Boffins develop interstellar alien ocean-spotting tool
'Pale blue dot' single-pixel planet problem cracked
Astronomy boffins say they have developed new techniques which could use the space telescopes of tomorrow to detect oceans on alien worlds orbiting other suns.
Earth as an alien invasion/harvest fleet would see it.
As all regular Reg readers will be aware, new telescope satellites now going into space are expected to discover large numbers of small, Earthlike planets in distant star systems. Some of these terrestrial-sized worlds are expected to be found orbiting at such a distance from their suns that liquid water could exist on their surface.
Liquid water is seen by most scientists as a prerequisite for the existence of alien life - at any rate, alien life of the same general sort as that on Earth; life which would be interesting on more than just an intellectual level. Hence the interest in finding liquid water elsewhere in the universe.
But simply finding a world on whose surface liquid water could exist without immediately boiling or freezing doesn't mean that there actually is any water there. And on the face of it, even the new space 'scopes now coming into service won't confirm the presence or absence of water. All they'll show, according to astronomers, is a "pale blue dot" less than one pixel in size, just as Earth itself appears when viewed from such far-ranging spacecraft as the Voyager probe (see pic).
The dot is pale blue for the same reason the daylit sky seems pale blue to us - scattering of sunlight by the atmosphere. Normally it's pretty difficult to tell from a single blue pixel whether or not any watery oceans lie beneath a planet's atmosphere, even if you know that it's theoretically possible for them to be there.
NASA boffins and their colleagues believe they've cracked this problem in advance, however, using images of Earth sent back from the Deep Impact comet probe, now on an extended mission in the outer reaches of our solar system.
"A 'pale blue dot' is the best picture we will get of an Earth-like extrasolar world using even the most advanced telescopes planned for the next couple decades," says Nicolas Cowan of the University of Washington. "So how do we find out if it is capable of supporting life? If we can determine that the planet has oceans of liquid water, it greatly increases the likelihood that it supports life. We used the High Resolution Imager telescope on Deep Impact to look at Earth from tens of millions of miles away - an 'alien' point of view - and developed a method to indicate the presence of oceans by analyzing how Earth's light changes as the planet rotates. This method can be used to identify extrasolar ocean-bearing Earths."
It seems that if you look at the right wavelengths it's possible to detect changes in an ocean-bearing planet's image as it rotates, even though different parts of the planet can't be distinguished from each other. The technique isn't perfect, apparently:
"We could erroneously see the planet as a desert world if it had a nearly solid band of continents around its equator and oceans at its poles," says Cowan. "[And] there are some weird scenarios you can dream up that don't involve oceans but would lead to varying patches of blue on a planet, but these are not very plausible."
The other snag with the new extraterrestrial ocean-spotting scheme is that it won't work with the present state of space telescope technology. It works fine using the Deep Impact scope to look at Earth, but over interstellar distances you'd need a scope better than anything now available. However, Cowan and chums believe that suitable kit will arrive in ten years or so, by which time there should be a fair number of known potentially-watery planets to check out.
There's more from NASA here. ®
I've read the paper and their result looks robust, but there's nothing really quantitative about how to apply it to exoplanets, there's basically just a throwaway sentence "we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope." Hmm.
OK then put in some numbers. Their imaging is done at ~0.5-1.0 microns, at which wavelengths the Earth is about 1E10 times fainter than the Sun, so at 10 parsecs the Earth would be about 30th magnitude. For this technique to work you need to image a few times a day to see the rotation - you can't just sit on the target for a long time to build up S/N.
So, ballpark, we want to get a 30th magnitude target at optical wavelengths in an hour. For comparison, the Hubble Deep Fields get to about ~29th magnitude at these wavelengths with 1E5 second exposures in each band, and a 2.5m mirror. So with S/N proportional to sqrt(time) that's about 27 mag in an hour with 2.5-m mirror, we want to go three mags deeper which is a factor of 15, which gives a ~10-m mirror (S/N proportional to D^2). That might be on the small side depending on how strict we are with our S/N requirement. Anyway - big, but not technologically impossible.
However this does assume that you can blot out the light from the 5th mag star 0.1 arcsec away!
One of the primary reasons that planet hunters are aiming for the IR rather than the optical is that the contrast between star and planet is "only" about 1E6 rather than 1E10. I'm not sure how easy it would be to divert their attention to short wavelengths...
give a German tourist a towel and sunbed, point at the watery contender of your choice and if he/she buggers off to get the best spot on the beach before breakfast then it's an oceanic planet. Simple, ja?
Mines the leather trenchcoat stuffed full of schizer videos.....
@ Liam Johnson / mirror size
Light collection should scale with area. To get a million times more area we need to scale radius up by thousands -- i.e. a mirror kilometers across. Much easier, but to do it in space might not be so easy unless you can make a mirror out of reflective film and rotate it to keep it spread out; whilst somehow controlling its shape. Oh yeah, with a separate space craft maneouvred accurately to the focus to take pictures.