Dr. Jonathan I. Lunine is professor of planetary science and physics and the chair of the theoretical astrophysics program at the University of Arizona. Dr. Lunine is also a distinguished visiting scientist at NASA's Jet Propulsion Laboratory, and an interdisciplinary scientist on the Cassini mission to Saturn and on the James Webb Next Generation telescope. He testified before the President's Commission on the Moon, Mars and Beyond on April 16, 2004.
I was told that my purpose here is to address the "beyond" element in the President's initiative and that's the "way beyond" element, apparently. I'm supposed to skip over the entire solar system and go directly to the search for Earths around other stars. And so I'll begin with a recurring dream that I had as a child growing up in the middle of New York City. In this dream, I was standing in an open field, night was falling, the stars came out in incredible brilliance. You don't see that in New York City except at the Hayden Planetarium. And the planets of our own solar system were there with unreal clarity. Suddenly, in this dream, I was moving through this cosmos seeing the stars that passed me, wondering what strange worlds awaited me at the other end of this journey. I never got to the other end before waking up.
|Jonathan Lunine of the Lunar and Planetary Laboratory at the University of Arizona
Image Credit: space.com
That's actually an appropriate metaphor for the human aspiration of finding other Earths around other stars. For four centuries before that, there were millennia of wondering and speculation and philosophy. Four centuries after the start of the Copernican revolution we still don't know whether a planet like the Earth exists in an appropriate orbit around another star. We know of 120 planets around other stars that are the size of Jupiter and Saturn, and those planets are abundant enough to tell us that the universe makes planets in abundance. It's an easy process; part of the star formation process. And most importantly, the technology for detecting Earth-sized planets is rapidly maturing.
We as a nation do know how to detect Earths around the nearest stars in the sky, the nearest stars to our own solar system, and therefore how to begin to implement the "beyond" part of the President's vision. It doesn't require astronauts; it does require putting into space potentially two different kinds of telescopic systems one that works at optical wavelengths and the other at infrared wavelengths. Our eyes work in the optical. The infrared we think of as heat radiation, but it is simply light of longer wavelengths than what our eyes can see.
So what are the advantages of both the optical and the infrared? Well, these contain different clues to the nature of a planet the composition of its atmosphere, its temperature variation, the presence of clouds. The infrared is better at determining temperature and the abundance of certain gases that we think of as part of a habitable planet. The optical part of the spectrum is better at looking for variability, looking for clouds. In the optical part of the spectrum, we can actually do this with a single telescope in space that's equipped with a device called a coronagraph that masks the light of the parent star.
|"We are witnessing the birth of a new observational science: the discovery and characterization of extrasolar planetary systems. " --G. Marcy, UC Berkeley Scene from a moon orbiting the extra-solar planet in orbit around the star HD70642.
Credit:David A. Hardy, astroart.org (c) pparc.ac.uk
(To detect Earth-like planets), the telescope would have to have a diameter at least six meters long; it could be shorter in width if need be, and it's within the capability of space-borne systems today. The other system would be an interferometer, which would work in the infrared. Coronagraphs don't work as well in the infrared; interferometers work somewhat better in the infrared. And here one would have several telescopes along a beam that would combine the light in a very precise way so that the star that one is looking at is blocked out completely, and that allows planets potentially to be detected from under the glare of that parent star.
There is what is called an interference pattern due to the combining of light from several mirrors in this telescope system, and an Earth is about a billion times less bright than the parent star. This device could be mirrors arrayed along a girder, or it could be free-flying spacecraft that are precisely aligned with each other. Its technology is less mature than the coronagraphic technology, and so the coronagraph is likely to fly first.
But with the two concepts, both called Terrestrial Planet Finder [TPF], we have the solid foundation for a program that would discover and characterize habitable planets around other stars before humans return to the Moon in force.
|The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
The coronagraph could fly in 2013 or 2014, perhaps, after a period of time here in 2007 to '11 when some precursor missions that are already on the books such as Kepler, SIM and next-generation space telescopes look for giant planets and also for Earths in close orbits around stars. The interferometer might fly in 2018. Now, I'm not advocating a plan that excludes astronauts, because really TPF, which is embodied by these two missions, is the gateway to something much grander, something that might require the intervention of astronauts in space or on the Moon.
If Earth-like planets are discovered around nearby stars, we'll want to know many things about them. Do they have continents and oceans? Is there plant life that's generating energy by photosynthesis?
(Answering these questions) requires a much larger device, which I'll call Life Finder, which is a large enough system that the light can be so finely divided that one could find the telltale signature of chlorophyll or other similar pigments associated with photosynthesis on this planet.
Conceivably one could resolve this planet, perhaps seeing details on the scale of half the size or a third of the size of the planet itself. I don't know what that type of system would look like. It's a long way off, two decades or more. And for that reason, the Commission should not recommend jumping directly to this very large Life Finder mission, which is a human-tended observatory on the Moon or somewhere in space for detecting and making the initial characterization of Earths around other stars. This can be done with TPF, and to try to do it with Life Finder would set the search back by a decade or more.
|Comparison of Mars, Venus and Earth in water bands, showing the clear presence of water on Earth uniquely
Credit: NASA Workshop, Pale Blue Dot
TPF could provide an incredible boost in interest in going to the Moon or sending astronauts to places like the stable Lagrange 2 point, if, in fact, TPF discovers that there is an Earth around a nearby star and there is then a strong desire to find out what the nature of that planet really is. So what I'm recommending is that over the coming decade the search for other Earths and the return of humans to destinations beyond Earth orbit take separate paths, with the intent that they meet again after both having seen their initial successes separately and not before.
I want to close by briefly sketching two versions of a story that takes place in 2014. A scientist takes her kids camping, and in version one NASA has not flown TPF because it's tied the search for other Earths to the establishment of a lunar base, and so nothing has flown yet. In the second version, TPF flies in 2014 and very quickly discovers, let's say, two Earth-sized planets around separate, nearby stars. In version one of the story, the family is sitting around the campfire; the kids ask their mother whether there are aliens in space and she replies that nobody knows. But she also says that there might be planets like our Earth spinning around some of the stars in the sky. So the family all looks up and the conversation ends as it always has through history in ambiguity, because no one ever knows if there are other Earths beyond our solar system.
In version 2 of this story, after TPF is flown, the same question is asked by the kids. But now the scientist walks their kids away from the campfire out into an open field and points to a certain set of constellations in the sky, and she points to two stars in particular and says, "Do you see these two stars? Each of them we know has an Earth orbiting around it, much like our own Earth orbits our sun. We know that there is air and there are clouds around that particular planet, the one around that star, and so there are plans to look more closely at it to see if there are signs of life." And then she concludes, "Maybe some day when your children's children's children are alive, they will go to that distant world to touch its soil and meet whoever or whatever is there."
No other generation before us in the whole history of humanity could lay claim to the second scenario. But we possibly can within a decade.
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