The other night I trained my telescope on a few stars that resemble the Sun and are now known to have planets—inconspicuous and previously unheralded stars such as 61 Virginis and 47 Ursae Majoris, each found to be orbited by at least three planets, and HD 81040, home to a gas giant six times as massive as mighty Jupiter.
I could see none of the actual planets—lost in the glare of their stars, exoplanets can only rarely be discerned through even the largest telescopes—but just knowing they were there enhanced the experience. Watching those yellow stars dancing in the eyepiece, I found myself grinning widely in the dark, like an interstellar Peeping Tom.
When I was a boy, the prospect of finding exoplanets was as dim and distant as the planets themselves. Theorists had their theories, but nobody knew whether planets were commonplace or cosmically rare. My 1959 edition of the opulent Larousse Encyclopedia of Astronomy noted that no planets of other stars had yet been identified, but predicted that “future instrumental and technical improvements may confidently be expected to reveal many things that are now hidden.”
And so they did. Thanks to space telescopes, digital cameras, high-speed computers and other innovations scarcely dreamt of a half century ago, astronomers today have located hundreds of exoplanets. Thousands more are awaiting confirmation. New worlds are being discovered on an almost daily basis.
These revelations advance the quest to find extraterrestrial life, help scientists better understand how our solar system evolved and provide a more accurate picture of how the universe—which is to say, the system that created us—actually works.
Two techniques are responsible for most of the planet-finding boom.
The transit method discerns the slight dimming in a star’s light that occurs when a planet passes in front of it. Some transits can be observed from Earth’s surface—even a few amateur astronomers have verified the presence of transiting exoplanets—but the technique came into its own with the launch in March 2009 of NASA’s Kepler satellite, a one-ton space telescope with a 95-megapixel camera that repeatedly photographs 150,000 stars in a single swath of sky off the left wing of Cygnus the Swan. Computers comb the images to find evidence of transits. The degree to which a star’s light is reduced (typically by less than one-thousandths of 1 percent) suggests each planet’s diameter, while the time the transit lasts reveals the size of the planet’s orbit. As I write this, the Kepler mission has discovered 74 planets; hundreds more are expected to be confirmed soon.
Doppler spectroscopy measures the subtle wobbling of stars—really surface distortions, like those of a tossed water balloon—caused by the gravitational tug of orbiting planets. When a star is tugged toward or away from us its light is shifted to slightly shorter or longer wavelengths, respectively, much as an ambulance siren sounds higher and then lower in pitch as the ambulance speeds past. The technique has revealed nearly 500 exoplanets.
Both approaches are better at finding massive planets orbiting close to their stars—the so-called “hot Jupiters”—than earthlike planets in earthlike orbits around sunlike stars. So it may be some time before planets that closely resemble Earth are identified, and even longer before astronomers can capture their meager, reflected light and interrogate it for the chemical signatures of life as we know it.
But it is the nature of exploration to find things different from what one expected to find, and the exoplanet hunters have unveiled planets quite unlike any previously envisioned. One is GJ 1214b, a “water world” more than twice Earth’s diameter that whips around a red dwarf star 40 light-years from Earth every 38 hours, its steamy surface boiling at an oven-hot 446 degrees Fahrenheit. The sunlike star Kepler-20, some 950 light-years away, has five planets, two of them comparable in mass to Earth, all packed into orbits smaller than Mercury’s around the Sun. WASP-17b is a big wisp of a world, about twice the size of Jupiter but only a tenth as dense, orbiting a star a thousand light-years from us.