Over the last three decades, astronomers have discovered more than 6,000 planets outside our solar system — yet none that look just like home. That might be about to change.
A new telescope project kicking off in the Canary Islands in mid-December is tasked with one main objective: to find an Earth twin. The Terra Hunting Experiment will be the first systematic quest dedicated to looking for Earthlike planets orbiting sunlike stars with sufficient resolution and observation time to stand a chance of making such a discovery.
It is a collaboration of a dozen research institutes led by astrophysicist Didier Queloz from the University of Cambridge, who shared the 2019 Nobel Prize in physics for the first discovery of any planet orbiting a sunlike star 30 years ago.
“We want to find a planet like Earth,” Queloz says. “The fact we don’t have any Earthlike systems is very frustrating.” Finding Earthlike planets is hard. The problem is that Earthlike planets have been nearly impossible to detect. That’s because detection methods can more easily spot huge planets that orbit close to their star.
Far-off planets are generally too faint to be seen directly, so astronomers rely on indirect measurements. One common technique, which will be used by the new survey, detects a star’s wobble as an orbiting planet gravitationally tugs it to and fro. To detect that minute wobble, researchers look for oscillating shifts in the wavelengths of starlight, which signal a star moving back and forth. These measurements, each of which takes as long as the observed planet needs to complete its orbit, also reveal a planet’s mass and the duration of its year.
A big planet orbiting a small star makes a big wobble. And closer, shorter orbits mean the wobble can be measured more frequently, allowing astronomers to quickly confirm the discovery. So, most exoplanets discovered so far are larger than Earth and orbit much closer to their star. They include gas giants called “hot Jupiters” and big rocky “sub-Neptunes,” and most are in very close orbit around an M dwarf — a type of small, dim star very different from our sun.
But for an Earthlike planet, the wobble is tinier and only completes approximately once a year. That means that confirming a detection can take many years, especially as the wobble signal gets drowned out by intrinsic larger fluctuations in light coming from the surface of the star. It’s like picking out a firefly in front of a campfire.
This limitation means we don’t yet know of any planetary systems that look like ours. “We have found no solar system equivalent,” Queloz says. “We are completely missing a big part of the story” of planetary systems.
