Key words :
?It?s just the excitement of the quest to not only find a planet that would be truly attractive and habitable, but one that has life?
26 Jul, 2006 04:07 pm
Webster Cash recently reported a new method to detect Earth-like planets around nearby stars using a space Telescope associated with a special shape occulter. He describes his finding for Scitizen.
On the cover of Nature we refer to “sister planets”. We’re looking for planets that have the same basic features as the Earth including oceans of liquid water. They have to be in the right temperature range, hopefully they will have lands as well.
Why detection of Earth-like planets is so challenging?
If you were to detect the Earth and pull it away from its sun but keep the brightness the same, our modern telescopes would be able to see it, in particular those like Hubble that are out in space, even in a distance like 30 light years. The reason why we haven’t found any Earth-like planets directly yet is because they’re only one Hubble resolution element away from their central star and that central star is 10 billion times brighter than the planet. This ratio of 10 billions to 1 is just overwhelming. The question is can we build a telescope or a system that separate star light from planet light cleanly?
Some techniques using ground telescopes, like gravitational microlensing, have been improved and quite successful these last years. Why do planet hunters need space telescopes?
Over the last 10 years, almost 200 planets have been found, most of them by the Doppler process in which we see the current star swinging around the center of mass of the orbit and that very small motion indicates indirectly that there must be another mass in the system. But we’re not looking directly at that planet. We’re just inferring its existence based on the laws of Newton. There are number of limitation to that. Fist of all you can’t study anything about the planet other than its mass and orbit. Secondly the smaller the mass of the planet, the harder it is to see, so you really have trouble detecting something like the Earth which is 500 times less massive than Jupiter.
When we see a star in the sky we want to go study it: we can measure its brightness, we can do spectroscopy to see what it’s made of, we can watch its motion… there are techniques that have been developed in the last centuries that allow astronomers to find out what’s going on in detail. Does the planet have water? We can see that in the spectrum. Does the planet have continents? You can probably see that in variations of the brightness in colors. So there is a lot to learn in direct observation of the planets separate from its parent star. That’s how we’re really going to open up the field of exoplanets study.
You say “in space, the problems of light scatter and diffraction in telescopes remain”. What are these problems?
The problem of light scatter and diffraction is not limited just to space telescopes. We have the same problems on the ground and even worse because we have scattering in the atmosphere of the Earth.
When you try to observe a star during the day time, the sun is so much brighter the light scattering in the atmosphere causes a blue light all over the place and you end up not being able to see the star. We have a similar kind of situation even in space when we try to look at a system directly. If you let the starlight into your telescope you have to make sure that it separates 1 part in 10 billion.
The idea of the starshade is to fly a second spacecraft with a big sheet of opaque plastic. It should be flown into the line of sight of the star to high precision, covering the central star with a disk and revealing the planet. Now the problem is that light undergoes diffraction. In essence what happens is when light goes nearby an object it doesn’t quite move in straight line, it bends around that object a small amount. The process of diffraction causes some of the light of the central star to move back to the center of the shadow you’re trying to create. That means you can’t get rid of all the central light with a conventional starshade shape. In fact this idea of starshades has been around for 3 or 4 decades. Various scientists have looked at how you could get rid of this, and up until last year nobody knew how to get rid of more than about one part in a hundred thousand, and that’s many orders of magnitudes short that what we need in order to see an Earth-like planet.
What solutions do you suggest?
The paper in Nature  describes a special shape for this starshade. It can operate at a certain distance, about twenty thousands kilometers away from the telescope out in space. Light will diffract around it but if we shape it just right it suppresses that angle change and you get a very dark shadow that goes to the factor of ten billion of more that you need. So it’s effectively an invention that allows you to create a deep shadow into which you can put a telescope and see the planets nearby.
In the paper, I mentioned the fact that we can build one of this starshade to work together with the James Webb Space Telescope. When you have two spacecraft that have to be held in very precise alignment with respect to a distant star, you can’t have one of them accelerating at high speed in a lower orbit like the Hubble telescope. Your telescope really has to go out way away from the Earth. The James Webb Space Telescope will be the first large telescope to be launched out that far. It’s scheduled to go a million miles away from the Earth. So, one of our ideas is to build a starshade that will follow the James Webb Space Telescope. Together the two of them can observe a planet like the Earth. That could happen as early as 2013. We would need to study this starshade for about a year and then it would take us about 5 to 6 years to build it, at which time the James Webb Space Telescope is supposed to be ready. That’s a nice coincidence of schedules. We have already proposed that NASA has a possibility.
Figure: schematic showing how a starshade in position against a nearby star might appear (Source: Nature).
Beyond that we really need to do a dedicated telescope: it would be 4 meters diameter and would operate in the visible band. The biggest problem with the James Web Space Telescope for this application is that it’s optimized to work in the infrared which is longer wavelength where there’s more diffraction. The result is it’s not really ideal. And if we did an ideal telescope, say a few years later, we would have so much power available that we could do spectroscopy of the planet out to 75 light years away and there are many interesting targets within that distance.
And then what would we see?
The excitement of the spectroscopy is that once you can break the light from an Earth-like planet into its component colors you can tell what molecules are in the atmosphere, so the first one you want to look for is water. Water has a very strong signature in the spectrum of the Earth, so we would expect to have a signature in any planet that has oceans. That’s step one. If you do find a planet with oceans, the next thing you want to look for is free oxygen. There is a signature in the atmosphere of the Earth that is very easy to see, that we could see in the planets at those distances that’s caused by the oxygen we breath. The oxygen in the atmosphere is only there because plants create it. If we can look for this bio-signature, there is a chance that we can establish the presence of life elsewhere in the Universe. Of course it would be highly controversial if we detected it because there may be other ways that we don’t know about put oxygen into the atmosphere.
It’s just the excitement of the quest to not only find a planet that would be truly attractive and habitable, but one that has life.
What is you’re feeling? Do you think we’ll find Earth-like planets around nearby stars?
What’s my feeling?... well… fifty-fifty! We’ve only been able to look at a few hundred star systems. I think there are probably lots and lots of planets like the Earth in the Universe. But my guess is that probably fewer than one percent of stars have them. I just don’t think they’re all over the place, everywhere you go. As a result it’s going to take us a long time. It may take several of these missions before we actually find that sister Earth. The older I get the more cynical I get! Probably when I was younger I would have said “yes we’re going to find them for sure”.
Webster Cash, thank you.
Interview by Gilles Prigent
 Nature, 442, 51-53 (6 July 2006)
Webster Cash works at the Center for Astrophysics and Space Astronomy at the University of Colorado, USA.