Searching for Other Worlds

by Bruce Moomaw

In their recent book, Rare Earth, Peter Ward and Donald Brownlee argue that although simple, durable one-celled life may be very common in the Universe, at most one nearby star out of several hundred may have any planet capable of evolving metazoan life, let alone intelligence. Given the vast number of stars within a few thousand light-years of us, though, it still seems very possible that our technology may allow us to identify some such stars. Even if no such worlds exist, we badly need to know much more about all the different types of planets around other stars in order to better understand just what the odds for complex life on them really are. Not surprisingly, there was considerable discussion of current and future techniques to detect extrasolar planets at the first Astrobiology Science Conference last month at NASA’s Ames Research Center.

Searching for Extrasolar Planets Today

Until this year, the only successful technique available to us was Doppler radial-velocity measurements. Any planet orbiting a star naturally tends to drag the star around with it in a slight wobble. This wobble is very slight given the tiny mass of all planets relative to their stars — Jupiter has only 1/1000 the mass of the Sun — which means current Earth-based telescopes can’t see the wobble directly.

An alternative means exists, though, to detect this wobble. The very sensitive spectrometers that we now have can detect the resulting very slight Doppler shifts in the frequency of the star’s light spectrum as the star shifts towards and away from the Earth. For five years now this method has been sensitive enough to detect Jupiter-mass planets closely orbiting fairly bright stars, allowing astronomers to determine these planets’ orbits and estimate their masses.

This technique requires very sensitive Doppler measurements, capable of detecting stellar velocity changes of only a few dozen meters per second. The smaller the planet, the smaller the Doppler shift — and so it was only in March that we were able to confirm the existence of the first two Saturn-sized extrasolar planets. The sensitivity of our Doppler measurements is increasing rapidly, though; D.J. Erskine described a newly-developed instrument capable of sensing a Doppler shift of only 1 meter/sec. The general feeling is that within a year, we’ll be able to confidently detect Uranus-sized extrasolar planets if they are close to their suns. But detecting planets as tiny as Earth (only 1/300 the mass of Jupiter) using this technique is, of course, far harder.

Besides mass sensitivity, the radial velocity technique has other limitations. A gas giant reasonably far from its star naturally has a long orbital period (12 years in the case of Jupiter), so you need both an observational period of decades to be able to detect the resulting very slow rhythmic change in the star’s velocity. The planets discovered to date have orbited very close to their parent stars; astronomers believe that at least five percent of Sun-like stars have such “hot Jupiters.”

Second, this technique can only set a lower limit on the planet’s mass — that is, a measure of what its mass is if the planet’s orbital plane just happens to be perfectly lined up with Earth’s line of sight to the star, so that the planet is dragging the star directly toward and away from our Sun. But, naturally, in reality such planets must have orbital planes tilted at all sorts of angles relative to our line of sight. Even a huge planet cannot be detected at all by the Doppler technique if its orbital plane is at right angles to our line of sight. So in reality, it’s a safe bet that our estimates of some of these planets’ masses must be seriously underestimated.

New Search Techniques

New techniques hold promise to detect more — and smaller — extrasolar planets. Astrometry, where the wobble in a star induced by a planet is directly observed, requires sensing position changes of as little as a few billionths of a degree, and is thus impractical with the Earth’s blurring atmosphere. Astronomy satellites, with much sharper eyes, may be better suited to utilize this technique. The small satellite FAME, scheduled for launch in 2004, will spend five years measuring the positions of millions of stars with an accuracy of only 14 billionths of a degree — enough to detect planets only twice Jupiter’s mass around several hundred stars within 75 light-years.

NASA hopes to launch the much bigger Space Interferometry Mission (SIM) in 2006, which will survey several hundred nearby stars with an accuracy 50 times better — good enough to detect Earth from a distance of 9 light-years! The greater sensitivity of these satellites will also enable us to detect giant planets much farther from their parent stars, allowing us to gauge just how unusual our own Solar System really is. Additionally, astrometry — unlike Doppler velocity measurements — will allow us to determine the “flatness” of the star’s looping path as seen by us, and so measure the tilt of the planet’s orbit toward us and thus its exact mass. Another promising near-future technique is transit observations, in which one simply measures the very slight decrease in the star’s apparent brightness as a planet repeatedly crosses in front of it — which about one-half percent of Earth-sized planets of other stars should do for us. This technique was uccessfully used for the first time last year to confirm an extrasolar planet (one of the hot Jupiters) — and it also allows determination of the planet’s diameter.

Laurence Doyle of the SETI Institute, in a conference poster, reported the results of his long-term transit survey of the binary red dwarf star pair CM Draconis, looking for a rocky terrestrial planet within its habitable zone. (Since the stars are so dim, such a planet would have a period of only a few weeks.) He could find no such planet down to a diameter of 31,000 km: “We believe this is the first survey of another stellar system for possible habitable planets to be performed.”

Like astrometry, though, transit observations can be done better from space. The French satellite Corot, to be launched in 2004, will make very sensitive light-fluctuation measurements of 6,000 stars, enabling it to look for large transiting planets. But Corot is a small satellite with different scientific goals, and is not really designed for this.

Another mission along those lines is Kepler, proposed by NASA’s Ames Research Center. Kepler is small and cheap enough that it has already been proposed for NASA’s low-cost Discovery program; it wasn’t selected the first time around but the proposal was well-received and will be resubmitted this year. Unlike Corot, FAME and SIM, Kepler would focus entirely on planet-hunting, spending four years observing a single field of 100,000 stars, checking all of the simultaneously for transiting planets.

Kepler would be able to firmly detect 150-200 Earth-sized planets in these stars’ habitable zones, and over 400 larger terrestrial-type planets, and thus, it might single-handedly give us our first really good estimate of how many alien planetary systems may contain planets that might support complex life.

Extrasolar Planet Pictures

Finally, there’s the obvious technique: trying to observe the planet directly. This is stupendously difficult — Jupiter would appear only one-billionth as bright as our Sun in visible light to an alien astronomer — but it, too, is teetering on the brink of feasibility, conference attendees reported. Bruce Macintosh described some of the first Earth-based attempts to directly observe planets during his conference talk. Large groundbased telescopes with adaptive optics systems, which correct for the distortions caused by the Earth’s atmosphere, combined with infrared detectors now make it possible to detect young gas giants — which are hotter and thus brighter at infrared wavelengths — at great distances from their parent stars. Right now the Keck Telescope, using its adaptive optics in the infrared, could spot a young Jupiter at 10 billion km from a nearby star, and a young planet five times Jupiter’s mass at 5 billion km distance. To date Macintosh’s search effort has turned up no such planets, perhaps because large planets do not form at such great distances.

Macintosh said that in the next few years, slightly bigger telescopes and longer observation times should allow detection of Jupiter-sized planets, at Jupiter’s 800 million km from its Sun, around the six nearest stars. The earth-based telescopes being considered for the next two decades — which will use computers to combine the images from dozens of separate mirrors to produce the equivalent of a single mirror with a diameter of 30-100 meters — should be able to directly see Jupiters around dozens of nearby stars.

Once again, though, space-based telescopes can be far better for this — if we can afford them. In interferometry, the longer the baseline separating two telescopes, the sharper the combined image that they generate is. The SIM mission will use pairs of small telescopes on a 10-meter boom. However, the Deep Space 3 mission, scheduled for 2005, will test placing a pair of telescopes on separate spacecraft and flying them in formation up to 500 meters apart. Such a mission demands extremely high accuracy — down to a few billionths of a meter — but recent techniques give us confidence that we can make it practical, even between distant free-flying satellites.

If a success, this technique may be applied to a followup to SIM, called the Terrestrial Planet Finder. It would interferometrically combine the images from several 3 or 4- meter telescopes (either mounted on a boom or flying in formation) in solar orbit, to cancel out all but a few millionths of the light from a star in a process called “interferometric nulling” first tested with groundbased telescopes in 1998. TPF could not only detect terrestrial worlds, it could measure the temperature, water and carbon dioxide on all Earth-sized planets within 50 light-years of Earth, thus determining their climate.

TPF would also try to detect traces of ozone in their air — for ozone is made from free oxygen, which can only exist in the air of an Earth-temperature planet if photosynthetic plants are churning it out in large amounts. It would also look for methane, which could probably exist in large traces in the air of an Earth-sized planet only if bacteria were producing it — thus allowing us to look for life on those planets where photosynthesis hasn’t yet evolved. It is thus quite possible that we’ll be able to solidly prove the existence of life on a planet of another star before we can prove or disprove it on Mars or Europa!

Astrobiology’s Future

Given the wildly enthusiastic reception that the conference got from the scientific community, a second conference — probably in 2002 — is a virtual certainty. National Astrobiology Institute Director Baruch Blumberg announced in his opening speech that there will soon be an editorial in the journal Science and a special issue of the Proceedings of the National Academy of Sciences on astrobiology, and a post-doctoral fellowship program run by the National Research Council. As he noted, astrobiology seems to have a remarkable ability not just to attract the interest of the general public, but that of scientists — and to allow those in different fields to combine their expertise in ways that aren’t often seen in today’s scientific world.