Exoplanet– a planet that orbits another star
The blocky space telescope was still getting its bearings, just a few months after launch, when the floodgates burst open.
As NASA’s Kepler Space Telescope science team was wrapping up a 10-day trial run, they saw something that bordered on the unbelievable: the telescope's first detection of a rocky, Earth-sized world outside our solar system.
The planet, a hot, heavy world dubbed Kepler-10b, would be among the early nuggets in a coming gold rush of exoplanet discovery—taking us from a handful of planets confirmed to be in orbit around other stars to more than 3,700 today, all in the space of two decades. Thousands more candidate planets found by Kepler await confirmation.
“In that trial run we saw, already, the signal of what could be a small planet orbiting a star about 540 light years away,” Natalie Batalha, an astrophysicist and member of the Kepler team, told a public radio host about the discovery, announced in 2011. “This was our first indication—‘Oh my god! We’re going to find lots of these things. We’re going to find lots of Earth-size planets.’”
Since the first confirmation of an exoplanet orbiting a sun-like star in 1995, and with only a few, narrow slices of our Milky Way galaxy so far surveyed, we’ve already struck many rich veins. A recent statistical estimate places, on average, at least one planet around every star in the galaxy. That means there's something on the order of a trillion planets in our galaxy alone, many of them in Earth’s size range.
“Right now we know, for the first time, that small planets are very common,” said Sara Seager, a professor at the Massachusetts Institute of Technology and an exoplanet research pioneer. “It’s phenomenal. We had no way to know that before Kepler. We’ll just say, colloquially: They’re everywhere.”
Hot Jupiters and wobbling suns
The planet discovered in 1995 was a hot, star-hugging gas giant believed to be about half the size of Jupiter. It tugged so hard on its parent star as it raced around in a four-day orbit that the star’s wobbling was obvious to earthly telescopes—once astronomers knew what to look for.
Finding this fast-moving giant, known as 51 Pegasi b, kicked off what might be called the “classical” period of planet hunting. The early technique of tracking wobbling stars revealed one planet after another, many of them large “hot Jupiters” with tight, blistering orbits.
The wobble method measures changes in a star’s “radial velocity.” The wavelengths of starlight are alternately squeezed and stretched as a star moves slightly closer, then slightly farther away from us. Those gyrations are caused by gravitational tugs, this way and that, from orbiting planets.
The European team of Michel Mayor and Didier Queloz announced their discovery of 51 Peg using this method in 1995, and the race was on to find others.
The others came—first by the dozens, then by the hundreds.
After confirming the existence of 51 Peg, a science team led by Paul Butler and Geoff Marcy, then of San Francisco State University, took a second look at data from their own radial velocity observations. They and the rest of the astronomical community hadn’t anticipated large planets orbiting so closely and rapidly around their parent stars. Sure enough, big, star-hugging planets began popping out of their data.
They announced two somewhat more plausible exoplanets, 70 Virginis and 47 Ursae Majoris, in 1996. The first had a 116-day orbit, the second an orbit of 2.5 years, helping overcome skepticism among their fellow astronomers; these distant solar systems looked a lot more like ours.
The Butler and Marcy team went on to discover at least 70 of the first 100 exoplanets in the decade that followed, attaining celebrity status. Scores of other ground-based research projects also joined the hunt, sending the tally of known exoplanets into the low hundreds.
Then a new space telescope, and a new planet-hunting method, stole the show.
Staring into space
Enter NASA’s Kepler Space Telescope, launched in 2009 to inaugurate what we could call the “modern” era of planet hunting. Kepler settled into an Earth-trailing orbit, then fixed its gaze on a small patch of sky. It stared at that patch for four years.
Within that small patch were some 150,000 stars. Kepler was waiting to catch tiny dips in the amount of light coming from individual stars, caused by planets crossing in front of them. The result: more than 2,000 confirmed exoplanets were sifted from the data, the bulk of the more than 3,300 confirmed so far, with more than 2,400 planetary candidates as scientists continue to mine Kepler’s observations.
The Kepler mission faced its own skeptical audience in the 1990s. Four times, NASA rejected the designs proposed by William Borucki of the NASA Ames Research Center in Moffet Field, California. Borucki, now retired, finally won approval in 2001.
His idea was proven right; Kepler’s four years of data are still revealing new planets. But failure of two reaction wheels on the spacecraft ended its primary mission in 2013.
Still, it’s hard to keep a good spacecraft down. The Kepler science team devised a clever fix: using the pressure of sunlight to stabilize one axis of the telescope. The instrument was rechristened "K2" and continues to discover planets, though at shorter observation times than its original four-year stare.
Other instruments, on the ground and in space, continue to round out the tally of exoplanets bagged so far. The European CoRoT satellite preceded Kepler, and also used the transit method to find numerous planets during its functional period from 2006 to 2012.
The Hubble Space Telescope not only has discovered a variety of transiting exoplanets, but has characterized the atmospheres of some of them. As a planet makes its transit across the face of its star, a sliver of starlight shines through the planet’s atmosphere. Gases and chemicals in the atmosphere absorb different wavelengths of the light as it passes through. By looking for these missing slices of the star’s light spectrum, scientists can tell which constituents are present in that alien atmosphere.
Another skywatcher, NASA’s Spitzer Space Telescope, observes transiting exoplanets in infrared wavelengths, and has helped to chart and characterize many, including puzzling out details of planetary atmospheres.
Spitzer often works in conjunction with ground-based telescopes, including OGLE’s Warsaw Telescope at the Las Campanas Observatory in Chile. In 2015, a collaboration between Spitzer and Italy’s 3.6 meter Galileo National Telescope in the Canary Islands revealed the closest known rocky planet: HD 219134b, only 21 light-years away from Earth. Disappointingly, however, the planet orbits its star too closely to make it suitable for life.
All but a handful of the thousands of exoplanets observed so far have been detected via indirect methods, such as watching for transits or measuring star wobbles. We’ve only just begun to enter a new era of planet hunting: direct imaging.
Hey exoplanets: Say 'cheese!'
Astronomers say the future of exoplanet exploration is all about direct observation. Missions like the James Webb Space Telescope, now under construction, and the planned WFIRST (Wide-Field Infrared Survey Telescope) will expand and sharpen our ability to capture actual images of distant planets.
New technology under development will boost these capabilities, allowing us to snap portraits of smaller and smaller exoplanets. The WFIRST mission, for example, will use an internal instrument called a coronagraph to selectively block and process incoming starlight to reveal the planets hidden in the glare.
Something similar could be done outside the telescope by a device called the starshade, being developed at JPL. The starshade would deploy in deep space like a sunflower the size of a baseball diamond. Tens of thousands of miles away, a space telescope would point toward it; the starshade would block unwanted starlight, allowing the space telescope to capture images of the planets around the target star.
In coming decades, as space telescopes grow larger and more refined, perhaps we’ll finally capture the iconic image of another Earth—a faraway world of continents, clouds and oceans.
But what, exactly, makes a planet Earth-like? Where should we look to find a habitable twin? To find out, read on to the next topic:
Human beings dream of alien life; NASA's looking for planets that have it.