Thanks to NASA’s pioneering Kepler probe, we know our galaxy is teeming with exoplanets. Now, a new generation of exoplanet hunters is set to home in on rocky worlds closer to home.
Over 9 years in space, Kepler has found more than 2600 confirmed exoplanets, implying hundreds of billions in the Milky Way. The new efforts sacrifice sheer numbers and target Earth-size planets whose composition, atmosphere, and climate—factors in whether they might be hospitable to life—could be studied. Leading the charge is the Transiting Exoplanet Survey Satellite (TESS), a NASA mission due for launch on 16 April.
The brainchild of researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, the $337 million TESS project aims to identify at least 50 rocky exoplanets—Earth-size or bigger—close enough for their atmospheres to be scrutinized by the much larger James Webb Space Telescope (JWST), due for launch in 2020. “Where do we point Webb?” TESS Principal Investigator George Ricker asked rhetorically at the American Astronomical Society annual meeting at National Harbor in Maryland in January. “This is the finder scope.”
Like Kepler, TESS finds planets by staring at stars and looking for a dip in brightness as a planet passes in front, blocking some of the star’s light in a so-called transit. But whereas Kepler kept a fixed view, watching just 0.25% of the sky out to a distance of 3000 lightyears, TESS will maneuver to observe 85% of it, out to about 300 light-years.
The spacecraft carries four telescopes that together will survey a strip of sky extending from the solar system’s pole to its equator, known as the ecliptic. The scopes will watch a strip for 27 days, then shift sideways and repeat the process. After observing 13 such strips over a year, covering almost an entire hemisphere of sky, TESS will flip over and survey the other hemisphere.
Over 2 years, TESS should measure the brightness of some 2 million stars, says project scientist Stephen Rinehart of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “If there is one planet per star [as Kepler predicts], we will see many. It’ll be a firehose of data.”
TESS’s primary targets are red dwarf stars, the most common stars in our neighborhood. Red dwarfs weigh less than half as much as the sun, so they do not burn brightly, offering several advantages to exoplanet hunters. A planet passing in front of a small, dim red dwarf blocks more of its light, yielding a stronger transit signal. Moreover, planets can whip around red dwarfs in orbits closer than Mercury’s, and still have hospitable climates. More orbits mean more opportunities for transit detections. TESS researchers are targeting speedsters that would circle the star at least twice during a 27-day TESS watch. Spotting two transits is key because it tells astronomers the length of the planet’s orbit. Other features of the transit—its duration, how much light is blocked, and how quickly the brightness dips—provide additional details such as the planet’s diameter.
Transits don’t reveal a planet’s mass, however, which is vital to determining its density—a clue to whether it is made of iron, rock, or ice. For this, TESS is relying on follow-up studies by ground-based telescopes, which can watch for tiny periodic Doppler shifts in the frequency of a star’s light caused by an orbiting planet tugging on it. The shift is a clue to the planet’s mass. Of the 5000 transitlike signals that the TESS team expects to detect, the clearest will be chosen for ground-based follow-up, says MIT’s Sara Seager, the mission’s deputy science director. The aim is to identify and weigh 50 planets to serve up to the JWST.
Although detecting planets around red dwarfs is easier, life may be less likely to arise there. Red dwarfs are erratic, prone to blasts of lethal radiation, and because the planets are so close, “they feel the effects of the star,” says astronomer Elisa Quintana of NASA Goddard. Close-in planets are also likely to be “tidally locked,” with one side always facing the star in an eternal scorching day while the other side freezes in an endless night. “Can they be habitable?” Quintana asks. “The debate goes back more than 10 years.”
Later this year, the European Space Agency will launch another eye on exoplanets: the Characterising Exoplanets Satellite. Rather than searching for new worlds, it will take a second, much longer look at transits of known planets to pin down their sizes more precisely. In combination with mass measurements from the ground, that should provide a better fix on planets’ densities.
Also debuting in the next few months is a ground-based search in Chile: SPECULOOS, the search for habitable planets eclipsing ultracool stars. The project’s four 1-meter telescopes have near-infrared sensors to detect transits of the very dimmest, coolest stars; a similar array in the Canary Islands will survey the northern sky. These stars are too faint for TESS’s small telescopes to see, but they could give the JWST valuable targets, says Michaël Gillon of the University of Liège in Belgium, which is leading the project.
SPECULOOS may be especially sensitive to small planets, because even small bodies will block noticeable amounts of light from the dim target stars. “TESS will find many more planets, but in the temperate—and potentially habitable—Earth-size regime, SPECULOOS’s detection potential should be significantly better,” Gillon says. “The next years are going to be very exciting!”