Astronomy Guide: Search For Extrasolar Planets

Information on the exploration of the solar system beyond the sun. IDeas about what kids are being taught now.

Humans have often looked to the stars for the answers to life's mysteries, and it didn't take our ancestors long to discover that the stars presented a few mysteries of their own. Although most stars were fixed, ancient astronomers soon realized that some moved across the sky on a nightly basis, much like the moon and sun but considerably more slowly. The ancient Greeks called these moving lights "planets," their word for "wanderers."

In ancient times, only five planets were known (Earth wasn't recognized as a planet until about 500 years ago). These "wanderers" included Mercury, Venus, Mars, Jupiter, and Saturn. The Eighteenth and Nineteenth centuries saw the discovery of two additional major planets, Uranus and Neptune, as well as a plethora of minor worlds called asteroids, which orbit mostly between Mars and Jupiter. The discovery of Pluto in 1929 rounded out the list of known solar planets. The hunt for the hypothetical Planet X continues, but recent discoveries and our current understanding of the way the universe works suggest that it doesn't exist.

In the Twentieth century, some astronomers set their sights on an infinitely more elusive goal: detecting stars around other planets. Actually imaging the planets was (and remains) entirely out of the question; the brightness of the parent stars would exceed any reflected light from the hypothetical planets by factors of millions or billions to one. Therefore, it was necessary for planet-hunters to use more devious ways to find their quarry. The two basic methods they adapted for this search were astrometry and spectrometry. Astrometry measures the movement of a stellar object; spectrometry measures the electromagnetic spectrum emitted by such an object.

The reason astrometry is useful in planet hunting is because the gravitational pull of an orbiting planet should slightly disturb the position of its parent star. Ideally, this wobble, though infinitesimal, can be measured. Similarly, spectrometry seeks to measure a star's Doppler shift -- that is, the potential shift in its electromagnetic spectrum caused by it being pulled toward and away from us by unseen companions. As a star moves toward us, its light shifts toward the blue end of the spectrum; when moving away, the light is red-shifted. Precise measurement and number-crunching of astrometric and spectrometric movement can reveal the size and number of a star's planetary companions. Unfortunately, for many decades planet-hunters were limited by their technology, and no planets were found.

Although there were a few false alarms in the middle part of the century, it wasn't until 1991 that astronomers detected extrasolar planets that survived scientific scrutiny -- though not by either of the standard methods outlined above. The star was a pulsar, the ancient remnant of a supernova -- certainly not the type of star around which one expects to find a planet. Pulsars rotate many times per second, producing an obvious and predictable natural radio signal. One particular pulsar with the inspiring name of PSR B1257+12 exhibited a slight irregularity in its signal, an irregularity best explained away by the presence of three Earth-sized planets circling the star and interfering a bit with its rotation. Given the harsh, radioactive conditions around the pulsar, any such planets would be lifeless balls of slag. It wasn't what the planet-hunters had been hoping for, but it was a start. Soon after, planets were detected around a second pulsar, PSR B1620-26.

It would be another three years before a planet was discovered around an ordinary, Sun-like star, and this time it was spectrometry that won the prize. In late September 1994, two Swiss astronomers, Michel Mayor and Didier Queloz, began to suspect that the star 51 Pegasi (the 51st brightest star in the southern constellation called The Pegasus) had a companion of planetary mass. By July 1995, they were sure of it. In late November, their paper announcing the find was published in Nature.

The find generated as much controversy as it did excitement. 51 Pegasi B, as the planet was named (the star itself is "A") wasn't exactly what most astronomers had expected. Planet-hunters had long since realized that they wouldn't be able to detect something as small as the Earth with their planet-bound telescopes, so the fact that the planet was about half the size of Jupiter was no surprise. What was mind-blowing was the fact that its year measured just over four days long -- indicating that it circled its star at a distance of 4.7 million miles, 20 times closer to its sun than the Earth is to ours. By contrast, Jupiter lies 484 million miles from the Sun, and takes 12 years to complete an orbit. The stellar system found by Mayor and Queloz defied all reasonable expectations of how a decent system should look, and left theoreticians scrambling to understand where they'd gone wrong. Nevertheless, just two weeks later, Americans Paul Butler and Geoff Marcy of California's Lick Observatory confirmed their discovery.

In the half-decade since 51 Pegasus B's discovery, it has survived all attempts to rationalize it out of existence -- and there have been numerous such attempts. The aftershocks of the discovery are still being felt; many astronomers have kicked their planet searches into overdrive. Teams led by Marcy and Butler, Mayor and Queloz, and others at several other observatories in Europe and the Americas have closely observed several hundred nearby stars since then. So far, their increasingly sophisticated astrometric and spectrometric techniques have brought the total of stars with known planets up to nearly 50, with 12-14 planetary systems waiting to be confirmed by additional observation. Several other stars are known to be surrounded by disks of material that may, in time, coalesce into planets. In addition, astronomers have discovered about a dozen stars that are accompanied by brown dwarves, tiny stars that either never ignited or ignited for a relatively short period before quietly burning out. At least one of these brown dwarves has been directly imaged, using an extremely sensitive camera coupled with instruments that blocked out the parent star's light.

By the late 1990s, astronomers were finding new extrasolar planets every few weeks. The surprises keep popping up: numerous "hot Jupiters" like 51 Pegasi B have been identified, proving that it's hardly unique, and in 1997 the star Upsilon Andromedae was found to have three Jupiter-like planets orbiting it -- the first identified multiplanet system of a normal star. Several multiplanet systems have been discovered since.

By mid-2000, more than fifty extrasolar planets -- "exoplanets" -- had been found, and in one case sensitive photometric (light-measurement) techniques have actually detected the transit of a large planet across the face of its parent star, HD 209458. Recently, it was discovered that Epsilon Eridani, which lies about 10 light years away (practically in our backyard), is orbited by at least one planet nearly as large as Jupiter.

Naturally, the discovery of entire planetary systems around other stars raises the possibility of some of those worlds harboring life. Given the conditions known for these systems, most would be unlikely abodes of life, but it's not unreasonable to expect some of these systems to possess planets or moons capable of supporting life. Unfortunately, any world possessing the proper conditions for any kind of life -- much less intelligent life -- would be undetectable at our present level of technology. Life as we know it should be possible only on worlds somewhat larger than the moon but far smaller than our system's gas giants, and we can't yet detect planets in this size range.

This may soon change. In the near future, planet-hunters will be able to detect stellar systems with an ever-increasing degree of accuracy; thanks to recently-built super-telescopes and new advances in optics, planets smaller than Saturn are already being detected. New facilities under construction should be able to detect planets as small as Neptune and Uranus. However, if we want to find Earth-sized worlds around normal stars, we'll need to go into space to do it. Much to the delight of eager astronomers everywhere, officials at NASA and the European Space Agency have proposed a variety of planet-hunting missions. NASA's planet-hunting missions seem unlikely to get off the ground anytime soon, but two such missions have been placed on the schedule by the ESA: GAIA, which will be launched by 2013 and will use astrometry to search for exoplanets; and Eddington, a "reserve" mission that will use photometric methods to search for Earthlike planets during stellar transits. In addition, the French space agency CNES has announced a mission called Corot, to be launched in 2004, which will use the transit method to detect Earth-sized planets.

Sometime later, more sophisticated satellites may be built well beyond Earth's orbit that will use advanced spectrometric techniques to not only detect Earth-sized planets, but to take a peek at the atmospheric makeup of such worlds. Water, ozone, and carbon dioxide, which together are indicators of biological activity, should be easily detectable in the spectra of such planets.

As children, many of us were told that there were only nine planets; a few of us can remember the days when only eight were known. But after decades of disappointment, we've finally established to our satisfaction that additional planets really do exist -- if only around other stars than our own. Now we can teach schoolchildren that there are not eight, not nine, but dozens of known planets, both in our solar system and elsewhere. If we're lucky -- and we probably will be -- in a few years we'll be able to teach them about other Earths out there, too.

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