Over the past three decades, astronomers have discovered more than 6,000 planets beyond our solar system, and yet none of them just like home. That may be about to change.
A new telescope project, kicking off in mid-December on the Canary Islands, has been tasked with one main objective: to find an Earth twin. The Terra Hunting Experiment will be the first systematic quest dedicated to finding Earthlike planets orbiting sunlike stars, with sufficient resolution and time to stand a chance of making such a discovery.
It's a collaboration of about a dozen research institutes led by University of Cambridge astrophysicist Didier Queloz, 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 all but impossible to detect. That’s because detection methods can more easily spot huge planets that orbit close to their star. Distant planets are mostly too dim to observe directly, and so far, astronomers have depended on indirect measurements. The most common technique, to be incorporated into the new survey, follows the wobble of a star as an orbiting planet gravitationally tugs it back and forth. To find that tiny wobble, researchers seek oscillating shifts in starlight wavelengths, a surefire sign that a star is jiggling backward and forward. These measurements, each taking as long as the observed planet requires to complete its orbit, can also reveal a planet's mass and how long it takes to complete one orbit around its star, the length of the year on that world.
This is because a large planet and a small star translate to a large wobble effect. Also, when their orbits are shorter, there is a high likelihood that their wobble effect will be measured, hence validating their discovery relatively fast. Thus, most exoplanets discovered so far are larger compared to planet Earth and also in very close proximity to their stars. These exoplanets comprise gas giants that are referred to as "hot Jupiters," as well as large “sub-Neptunes,” that are in very close orbit around an M dwarf, a small, dim star compared to our sun.
However, for an Earth-like planet, the wobble is much smaller and takes about one Earth year to complete. This implies that it may take several Earth years for the confirmation of detection, especially when the signal is overpowered by natural large light variability emitted from the surface of the star. It’s like trying to spot a firefly lighting up in front of a bonfire.
This means that we don’t know any planet–system resembling our own yet. “We have not discovered any equivalent to any solar system,” explains Queloz. We are missing the whole other half of the picture when it comes to planet systems.
Earth-like planets will help in the search for E.T.
As frustrating as this knowledge gap is for astrophysicists, this unknown information also obstructs research towards the detection of extraterrestrial life.
Astrobiologists study the starlight lapping at the edge of a planet’s atmosphere for traces of biosignatures – compounds that might indicate the presence of life. However, the viability of life in the planetary systems currently known remains inconclusive due to the immense differences between them and our system. Take the M dwarfs, for instance – they release massive bursts of radiation that would bombastically incinerate any adjacent planet. Furthermore, the close orbits of these planets would naturally cause them to be tidally locked, with one side in perpetual sunlight.
But and besides, writes Queloz further: "It's actually way more difficult if you're looking at completely different kinds of planets.”
This is exactly why he and others think that our best hope in searching for life elsewhere in the universe will most likely be on an Earth-sized planet that orbits in the habitable zone around a star somewhat like our sun itself; this is what astrophysicists term a G dwarf or a slight K dwarf. "If we are looking for life," asserts astrophysicist Clark Baker of the University of Cambridge, "I think our best hope is on an Earth-like planet."
A fresh search for planets similar to Earth is soon to get underway
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Queloz cites a lack of "Earth-like" discovery as being down to a "technical wall" in being unable to look past the "noise" created by stars. However, after a decade of research, he claims to have finally solved it. Their tool is HARPS3, a world-class spectrograph – a device that unravels the colors in starlight, or rather, the wavelengths – built to detect the velocity of stars relative to our solar system with a level of precision to detect planets against all other background noise. They installed the HARPS3 on a telescope that was constructed in the 1960s as a project for the Royal Greenwich Observatory in England but later relocated to the Canary Islands’ La Palma in the 1980s. “It was many factors cheaper to refurbish than build a new telescope like this,” Baker explains. “It was mechanically perfectly good. It was reliable and precise. It was ideal for the survey.
To have such a rich history telescope as this for our survey is just lovely,” he says. The telescope has the capability to measure the motion of stars accurately at 10 cm/sec, that being roughly the pace of a baby crawling, according to Baker. While accuracy is paramount, it is not the only key consideration. The stars that the project will choose for their nightly observations for the next 10 years will be the same group of up to 50 stars that are similar to the Sun. They chose their stars based on criteria that included being similar to the Sun but offering high-quality observations.
“You have to be able to measure every night over a decade in order to have enough data to beat down the noise and find those Earth twins,” Baker explains.
“It was very spooky to put 10 years of my life and equipment worth £10 million into these 20-foot containers to ship to La Palma in October," says Cambridge astrophysicist Samantha Thompson, who hopes this study will “find good evidence of something like Earth.” The concept of a “‘pale blue dot’ from another solar system” sends shivers down the spine of astrophysicist Samantha Thompson, who is discussing the famous photo of our home planet, taken by the Voyager 1 spacecraft in 1990 as it departed our solar system to head into interstellar space.
'Terra Hunting' represents one of the efforts being made to tackle the problem of climate
estimates to discover at least two Earth-like planets in its habitable zone, where water can flow. These planets would be prime candidates for future missions to observe for signs of life.
All these estimates come based on information gathered by NASA’s Kepler mission, where 2,600 planets have been discovered in its near 10-year mission.
Sarah Rugheimer, an astronomer at the University of Edinburgh who was not involved in the study, tells *Scientific American* that the research "fills an important region of parameter space that is lacking in our exoplanet hunt," in part because it searches for planets that orbit stars like our own sun and have larger orbital periods.
“This mission has the potential to bring us closer towards finding ‘the strongest evidence of life’ that has existed outside of Earth,” adds another member who is not part of the Terra Hunting team, Paul Rimmer, an astrochemist from the University of Cambridge, who is also not part of the Terra Hunting team. Upcoming missions like the Habitable Exoplanet Observatory from NASA or the Large Interferometer for Exoplanets set up in Europe would then focus on these planets that would be spotted by the Terra Hunting Experiment.
For Queloz, the initiative represents a test case. “If they work,” he says, “we will certainly need more of them.”
A search of this kind has already been proposed for the southern sky by a team of Danes, to be conducted with a new instrument in Chile that will look forEarth−like planets over five years. LarsBuchhave, the leader of the search, an astrophysicist at the Technical University of Denmark in Kongens Lyngby, says that his search, in the southern hemisphere, will complement their search, in the northern hemisphere, called Terra Hunting.
Queloz concurred. “Finding an Earth-like planet,” he said, “is such a challenging task, it requires a worldwide effort.”
But what happens if Terra Hunting doesn't turn up anything in 10 years' time? “That would be an equally interesting finding,” Queloz continues, “We will be more special than we think.” The author is on a media fellowship at the University of Cambridge sponsored by the European Molecular Biology Organization.
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