Astronomers estimate 29 potentially habitable exoplanets may have received signals from Earth
7 star systems are within 100 light-years of Earth
The Milky Way is a big place. There are an estimated 200 billion stars, most of them with one or more planets. As a result, in the search for extraterrestrial life, it's like looking for a needle in a haystack.
However, a new study has released a list of potentially habitable planets that may have already received a signal from Earth, may be receiving them now, or will within 5,000 years.
The study, published in Nature, used data collected by the European Space Agency's (ESA) Gaia space telescope, which is mapping our galaxy in unprecedented 3D.
In our search for exoplanets, or planets orbiting other stars, one of the most popular methods is the transit method, where the light of a star dips ever-so-slightly (maybe close to 1 per cent), which indicates a planet has crossed in front of — or transited — the star.
However, this method requires the planet to be in our line of sight. It's like someone holding a pea in front of a giant light bulb; if it were held anywhere above or below the light bulb, we wouldn't notice the slight dip in brightness.
The authors of the new study found that, over the past 5,000 years, roughly 1,715 nearby stars would have been in a position to see Earth in this transit method, and of those, 29 potentially habitable planets around those stars could have also received human-made radio transmissions, which began roughly 100 years ago. That is, if they know to look for radio signals.
WATCH | Astronomers estimate number of star systems that may have or could detect life on Earth:
But there are a few stars that we know have potentially habitable planets around them: Ross 128; Teegarden's Star; GJ 9066; TRAPPIST-1, K2-65; K2-155 and K2-240.
"So we have in the set seven stars that have planets, and four of those are within 100 light-years," said Lisa Kaltenegger, lead author of the study and associate professor at Cornell University in Ithaca, N.Y.
"So which ones could see us transit, plus, would they have already gotten our radio waves that we started to send out about 100 years ago? And that's the whole question: does everybody look for radio waves? Does everybody develop radio technology? Nobody knows."
One of the most interesting is the TRAPPIST-1 system of seven exoplanets, three of which are rocky and lie in the potentially habitable zone (where water can exist on a planet's surface).
However, TRAPPIST-1 won't see us for another 1,600 years, Kaltenegger said.
"So we already found them," she said. "But their vantage point has not gotten to this perfect spot yet to see us transit. So in a way we know something they don't for 1,600 years."
Ross 128 is a red dwarf that lies roughly 11 light-years away, which means it is close enough to receive Earth broadcasts (radio signals travel at the speed of light) and has a large planet that's almost twice the size of Earth. If there's life there, they would have seen Earth transit for more than 2,000 years. However, 900 years ago, they lost that point of view of Earth.
But Teegarden's star, which lies 12.5 light-years away, will be in a great spot in 29 years to receive Earth's transmissions.
To broadcast or to not broadcast
When it comes to the discussion of the search for extraterrestrial life, the big question is whether or not we want to be broadcasting our existence out into space: What if we alert an intelligent but not so benevolent species to our existence?
However, despite the debate, there is still a co-ordinated effort to search for extraterrestrial intelligence, known as SETI, mainly by the SETI institute and the Breakthrough Listen initiative.
"What [the authors] have done here is they've sort of listed all the star systems that could see Earth passing in front of the sun that are relatively close to us," said Seth Shostak, senior astronomer at the SETI Institute, who was not involved in the study. "So they've given SETI a whole bunch of targets."
While signals have been sent out into space from our radio transmissions, they're not directed, but rather something astronomers term as "leakage."
"If they have the kind of antennas we do, they're not going to pick up this leakage radiation as it's called from Earth, right? Because … it's not deliberately aimed at them," Shostak said.
The only directed signal was sent out in 1974 by the now defunct Arecibo telescope in Puerto Rico. But it was sent to a star cluster, M13, that lies more than 21,000 light-years away.
But so far, it's been quiet. The closest we ever came to a potential signal was in 1977, often referred to as the "Wow! signal." It lasted for 72 seconds, and then disappeared. Since then, there has been much skepticism about it, with one paper claiming that it was the result of passing comets.
Nick Cowan, an astronomer and associate professor at McGill University's department of Earth and planetary sciences who was not involved in the study, said that there are many scenarios that may account for the silence.
"I mean there's a bunch of resolutions to that, which range from terrifying, like the moment you start leaking radio into space within however long it takes for the aliens to get here, they will get here and exterminate us, to the really sort of depressing," he said. "Like maybe intelligent life is actually just exceedingly rare, and it's arisen on Earth through some series of evolutionary flukes."
This leads to the Fermi Paradox, named after physicist Enrico Fermi who is said to have asked, "Where is everybody?" at a lunch table with other scientists at the Los Alamos National Observatory in 1950, decades before we discovered exoplanets. The idea is, if there are so many stars, there should be millions or billions of planets, and potentially intelligent life that we should be able to detect. But so far, we have been greeted by a great silence.
But the search still goes on, and this recent study might help narrow down some of the targets.
"Sometimes when I look up at the sky now I wonder," said Kaltenegger. "There are [about] 2,000 stars that could be seeing us right now and I'm an optimist, so I'm like, the sky just got a little bit friendlier because I imagine if life were out there it would be waving, you know? It's like, 'Hey, are you out there?'"
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