In astronomy, some words start out as proper nouns: Sun, Moon, Earth, Jupiter. We realized that other stars are also glowing balls of gas like our Sun, and so we talk about the night sky as being full of suns. We also learned that other planets have moons like ours, and so we have our Moon and dozens of moons elsewhere in the solar system. I think that soon we will see lowercase earths and jupiters, as we can talk with more and more confidence about planets around other stars, and compare them to the planets around our local star.
The question of whether or not there is intelligent life on other planets is an important one for us as thinking meat. We want to know if there are other thinking beings out there, or if we are alone in the cosmos. Written records of humankind’s thoughts on the question go back to the early Greek philosophers, although we have taken different attitudes toward it over the centuries. We are finally in a position to at least discover whether or not life exists elsewhere in the universe, if not to locate intelligent life for sure.
Early questioners had no way to discover what was going on even on the surface of the moon, a mere quarter of a million miles away, much less what kind of planets might be in orbit around distant stars. It took us several centuries of telescopic observing before we had identified all the planets of our own solar system, and then suddenly at the end of the twentieth century we entered an amazing period of discovery as astronomers sought and found evidence of planets orbiting other stars. So far they have identified 154 planets around normal stars, including 14 planetary systems of more than one planet, plus some possible planets orbiting pulsars. (For the latest numbers, see the Catalogue of Extrasolar Planets.) What we’ve been able to detect so far from Earth is mostly large planets; the smallest one so far around a normal star is 14 times the size of Earth, but it’s in a very unearthlike orbit (very close to its parent star). Astronomers believe it’s likely that they have even gotten an image of an extrasolar planet, more massive than Jupiter and orbiting a brown dwarf, and another image of a planet orbiting a very young star. (However, the latter might turn out to be too massive to be a planet.) [April 30, 2005: The European Southern Observatory has issued a press release confirming that the former is indeed an extrasolar planet orbiting the brown dwarf.]
NASA has three missions in the works that will follow up on and complement the ongoing ground-based discoveries. These missions will use a variety of techniques to search for extrasolar planets, in particular those that are similar to Earth and are in the habitable zone for their parent star (i.e., at a distance where liquid water is possible). Furthermore, these missions should increase the amount of data we have about planetary systems: for each system, how many planets there are and their sizes and orbits.
With these missions and ongoing ground-based discoveries, we’ll have the basis for a broad comparative study of planetary systems. The planetary systems discovered so far from the ground do not resemble our own very closely; the big planets in these systems are generally much closer in to their stars than the big planets in ours. As we have more data to work with, we can begin to figure out how planetary systems form and how many of the various types there are in the nearby universe. As thinking-meat achievements go, this one is shaping up to be spectacular.
The Kepler mission will look for the ever-so-slight but regular dimming of a star when its planets pass in front of it, or transit. Kepler is scheduled to launch in October of 2007, and will watch 100,000 stars for four years, looking for tiny repeated dips in starlight that should occur if a planet is periodically blocking part of its star’s light as it transits the star.
It takes a very sensitive photometer to measure this decrease in starlight; Kepler’s photometer will be able to measure a change in brightness of about 100 parts per million. Once such a dimming is observed, we can calculate the planet’s orbit from the length of time that the star’s light is decreased. By measuring how much the star’s light is decreased, we can calculate the size of the planet.
Keep in mind that the further out a planet is, the more slowly it circles its star. If you were viewing the solar system from far away and edge on, you’d see the Earth cross the face of the Sun every year; Mars, which is the next-furthest planet from the Sun, would transit every two years. By observing the same stars continuously for four years, Kepler would be able to see four transits for an Earth-like planet orbiting in the habitable zone of a Sun-like star, and fewer transits for planets further out. Kepler can find planets as small as Mercury, but is optimized to search for Earth-sized planets. Kepler should give us a better idea of how many stars have terrestrial planets.
If Kepler finds any terrestrial planets, we can look more closely at them with later missions. Kepler should also give us a much broader data set for comparing the planetary systems.
SIM PlanetQuest, which is slated for launch in 2011, will look for the telltale wobble, very tiny but detectable, of a star being tugged on by its planets. Since the star is very large compared to the planet, the gravitational influence that the star has on the planet is much much greater than the influence that the planet has on the star. However, if you look carefully enough, you can see that a star wobbles very slightly but regularly as the planet orbits it.
(Many of the ground-based discoveries, by the way, are based on detecting this same small wobble. From the ground, however, what researchers look for is very small periodic Doppler shifts in a star’s spectrum, redward and blueward as the star moves away from and toward us.)
SIM stands for Space Interferometry Mission, the original name for this mission. Interferometry is the process of combining light from two or more telescopes to simulate a single telescope of much bigger light-gathering capacity. The Very Large Array (VLA) of radio telescopes in New Mexico uses interferometry, which is easier to do with longer waves like radio waves. To do it with the much shorter wavelengths of visible light, it’s best to go into space. SIM PlanetQuest will use visible-light interferometry to measure the positions of stars much more accurately than we have been able to so far, and the accuracy of this positioning will also enable us to search for the signs of planets around these stars.
SIM PlanetQuest will look at the closest 250 stars in search of terrestrial planets. This is the hardest part of SIM, and will require the most precise measurements (one-millionth of an arcsecond, which is the thickness of a nickel on the moon as seen from the Earth). SIM will also survey a larger group of about 2,000 stars with a lower accuracy, looking for Neptune-sized and bigger planets to try to figure out how many of them there might be in this neck of the galactic woods. A third goal for the mission is to look for Jupiter-mass planets around young stars. The fact that so many of the planetary systems we’ve discovered from Earth are so different from our own has sparked a great deal of curiosity about how these systems and their planets are formed. By looking at young systems, the SIM mission might be able to shed some light on the early development of planetary systems.
The Terrestrial Planet Finder (TPF) is the most ambitious of all the missions. It has two complementary parts: an interferometer that works in the infrared (not visible light like SIM), and a coronagraph. The idea behind the coronagraph is to observe planets directly by blocking the light of the parent star so that the comparatively dim planets have a chance to appear. Since the star is very bright and the planet very dim in comparison, this is a daunting task, and the smaller the planet, the harder it will be. (Note that the extrasolar planets possibly imaged so far from Earth were more massive than Jupiter.)
TPF, scheduled to launch between 2014 and 2020, will study up to 150 stars within 45 light years of Earth. In addition to detecting terrestrial planets, TPF would also analyze the spectra of any planets it finds. By breaking a planet’s light down into its component colors, TPF will reveal the chemical composition of the planet. This is where it gets extremely exciting, because in addition to detecting a planetary atmosphere and telling us what it’s made of, TPF can also look for biomarkers like the presence of molecular oxygen and methane, and also for chlorophyll.
If you could make such a spectrum for Earth, you would compress all of this planet’s biodiversity, all of the living processes, into a single diagram, dense with information but obviously not a complete picture of the planet. What would it be like to see a similar diagram for a totally alien world? If we have the luck to look in the right place, I wouldn’t really be surprised to see such a life-revealing spectrum. Earth harbors life in such unlikely and downright hostile places that I suppose that other planets might well harbor at least simple forms of life, although I’d guess that highly complex, intelligent beings are not very common. But who knows what we will find?
Two European missions, Corot and Darwin, will also look for extrasolar planets and signs of life.
Why should we look? I’m not sure it provides economic justification that would satisfy a financial analyst, but my answer is that I can’t imagine us not looking. It’s part of what we do best. Humans are curious about their environment. Maybe this is an adaptation; curious hominids might well do better at surviving and reproducing than slack-jawed dullards who lack the energy or inclination to see what’s over the next hill or whether there’s an easier way to do something. Deep space is not our immediate environment, true, and it doesn’t have much to contribute to our capacities for survival or reproduction (I don’t believe that space colonization offers much hope for relieving our resource problems on Earth, for example). But I don’t think we can turn our curiosity off; even if it evolved as an aid in certain specific situations, it’s a part of us now that we tend to apply in all kinds of other situations.
And the question of whether we’re unique or not in the universe is a very fundamental question, although on a huge scale, about our place in the natural environment. The discovery of even simple life on another planet would be bound to cause deep changes in our worldview. (In addition to the jubilation and excitement, and the possibility of a new science of comparative biology, I can imagine the conspiracy theories already, saying that the observations are faked. I just wish I had the imagination to weave a good story about why people would say they are faked, so I could write a blockbuster novel and make big bucks.)
Geoffrey Marcy, who with his team discovered over half of the extrasolar planets we know so far, recently gave a series of talks at Indiana University that provided an excellent look at the state of extrasolar planetary science. (If you ever get a chance to hear him talk, by all means go!) In particular, he gave a compelling presentation on why we should look for other life in the universe.
One of his points was that we have some inherited traits that are counterproductive for us now (e.g., xenophobia and territoriality), but we also have some favorable attributes like curiosity and the capacity for compassion, and it’s an open question which aspects will prevail. I think that our hope as a species lies in nurturing the favorable traits, in particular curiosity. If exploration is one of the things we humans do, then let us do it well.
Looking for other life in the cosmos implies a belief in our own future—why bother looking if we believe that we ourselves are to be extinguished soon because of our own environmental and political mistakes? Furthermore, if we value life enough to seek it out among the stars, to me that goes hand in hand with valuing it enough to try to preserve it, in all its diversity, down here on this planet. There are other ways we can express both our curiosity and our belief in the future, but this is one of the important ones. May the meat keep on thinking, long into the future.
