A new hunt for an Earth analog begins

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.

Earthlike planets can guide the search for E.T.

This gap in knowledge is not just frustrating for astrophysicists, it also hampers the search for extraterrestrial life.

Astrobiologists examine starlight skirting a planetary atmosphere in search of biosignatures — molecules that could suggest the presence of life. But whether life can exist in the planetary systems we know of so far is a matter of debate as they are so different and therefore difficult to compare with our solar system. For example, M dwarfs unleash frequent, violent bursts of radiation that might repeatedly scorch nearby planets. Plus, the proximity of such planets to their star means their rotation is tidally locked, exposing just one side to eternal daylight.

And besides, Queloz notes, looking for life in space is “way more difficult” on unfamiliar types of planets.

That’s why he and others believe that our best bet for finding alien life is on an Earth-sized planet in the habitable zone of a sunlike star, what astronomers call a G dwarf or a slightly cooler K dwarf. “If we’re going to find life, my bet is it’s going to be on an Earthlike planet,” says Clark Baker, an astrophysicist at the University of Cambridge.

A new search for Earthlike planets is about to kick off

Queloz blames the lack of Earthlike discoveries on a “technical wall” of not being able to see past the noise produced by stars. But after a decade of development, he and his team think they’ve cracked it.

Their solution is HARPS3, a state-of-the-art spectrograph — an instrument that breaks starlight into its component colors, or wavelengths — designed specifically to measure the velocity of stars relative to our solar system with high enough accuracy to pick out planets amidst all the noise.

The team attached HARPS3 to a telescope originally built in the 1960s for the Royal Greenwich Observatory in England and moved in the 1980s to La Palma, in the Canary Islands. “It was many factors cheaper to refurbish than build a new telescope of that class,” Baker says. “Mechanically, it was perfectly good: reliable, precise and great for this survey.

To have a telescope with this sort of history to do our survey is a lovely thing.” The instrument can measure stellar motion of about 10 centimeters per second — roughly the speed of a baby crawling, Baker says. But precision alone isn’t enough. The project will observe the same set of up to 50 sunlike stars every night for 10 years, operating fully robotically to make the observations less laborious. The team picked the stars based on a range of parameters to focus on those most similar to our sun but also most likely to provide good quality data.

“You need to measure every night over a decade to get enough data to beat down the noise and detect those Earth twins,” Baker says.  

Cambridge astrophysicist Samantha Thompson describes the unnerving feeling of putting “10 years of my life and £10 million worth of equipment into 20-foot shipping containers” for transport to La Palma back in October. She hopes the experiment will “find good evidence of something like Earth.” The idea of a “‘pale blue dot’ from another solar system” makes her feel “all shivery,” she says, referring to the iconic image of Earth taken by NASA’s Voyager 1 space probe in 1990 on its way out of the solar system.

‘Terra Hunting’ is one part of a worldwide effort

Based on data from NASA’s Kepler mission, which found about 2,600 exoplanets during its nearly 10-year-long mission, the team expects to discover at least two Earthlike planets in the habitable zone, where liquid water can exist. Those worlds would then be prime targets for planned telescope missions to look for signs of life.

Sarah Rugheimer, an astronomer at the University of Edinburgh not involved with the experiment, says the project “fills a critical parameter space that’s currently missing in our exoplanet search,” particularly by looking for planets with longer orbital periods around stars more like our sun.

University of Cambridge astrochemist Paul Rimmer, also not part of the Terra Hunting team, says the project has potential to lead us to the “strongest evidence of life” beyond Earth yet. Future missions such as the planned NASA Habitable Exoplanet Observatory or the Europe-based Large Interferometer for Exoplanets could zoom in on planets discovered by the Terra Hunting Experiment, studying their atmospheres for biosignatures.

Queloz sees the project as a test case. “If it works, we’ll need way more of them,” he says.

Already, a Danish team is planning a similar five-year search for Earthlike planets using a new instrument in Chile that will survey the southern sky. Project lead Lars Buchhave, an astrophysicist at the Technical University of Denmark in Kongens Lyngby, notes that having their effort in the south and Terra Hunting in the north makes the two projects complementary.

Queloz agrees. The search for an Earthlike planet, he says, is “such a challenging goal, it needs a global effort.”

And if Terra Hunting finds nothing after 10 years? “That would be a fascinating result too,” Queloz says. “We will be more special than we think.”

The author is visiting the University of Cambridge on a media fellowship funded by the European Molecular Biology Organization.