There could be as many as 29 potentially habitable worlds “in an ideal position” to observe Earth, if there were an intelligent civilization, according to a new study.
To explore ways to find exoplanets, worlds outside the solar system, the research team from Cornell University reversed the process to see which ones could detect us.
Working alongside the American Museum of Natural History in New York City, astronomers have identified 2,034 star systems in our galactic region – within 326 light-years of Earth – that can view our planet.
Of these stars, about 1,715 have been possible to observe the Earth since human civilization flourished about 5,000 years ago, and 319 will be added over the next 5,000 years.
The number of stars varies depending on their position in space in relation to the solar system, and changes due to the fact that we live in a dynamic world.
While not all exoplanets have been discovered around the stars that can observe Earth, the team estimates that 29 years will have a rocky world in the habitable zone, which is also well positioned to detect radio waves from humans over 100 years ago.
It can be argued that current telescopes are not able to detect signs of life inside the atmosphere of an exoplanet, or whether they are really habitable, but future observatories, including NASA’s James Webb Space Telescope, will delve deeper into these distant worlds than ever before.
“From an exoplanet standpoint, we’re extraterrestrials,” said Lisa Kaltenegger, professor of astronomy and director of the Carl Sagan Institute at Cornell. “We wanted to know which stars had a proper view of Earth, because they block sunlight.”
As stars move around in our dynamic universe, this vantage point is gained and lost over time, so they were able to identify specific star systems.
The transit method is one of the main methods astronomers use to discover planets in other star systems – they look for “dips” in the light from the star.
This requires determining the exact location of the exoplanets, the star that orbits it and the Earth, so that we can see it as it passes and this changes over time.
Kaltenegger and astrophysicist Jackie Ferti, a senior scientist at the American Museum of Natural History, used positions and motions from the European Space Agency’s Gaia catalog to identify stars entering and leaving the Earth’s transit zone.
“Gaia provided us with an accurate map of the Milky Way, allowing us to look back and forth in time and see where the stars were and where they were heading,” Verti said.
Of the 2,034 star systems that pass through the Earth’s transit zone over a 10,000-year period, 117 are within 100 light-years of the Sun.
Among them, 75 objects have been in the transit zone of the Earth since commercial radio stations on our planet began broadcasting into space about a century ago. In the catalog of 2034 star systems, seven are known to host exoplanets. And each of these worlds has or will have a chance to discover Earth, just as Earth scientists have found thousands of worlds orbiting other stars.
And if the exoplanets were attracting intelligent life, they could observe the Earth backlit by the sun and see chemical signs of life in our atmosphere, including oxygen.
Ross 128, with its red dwarf host star located in the constellation Virgo, is about 11 light-years away and is the second-closest system with an Earth-sized exoplanet – its world is about 1.8 times the size of our planet.
The Trappist-1 system, 45 light-years from Earth, hosts seven transiting Earth-size planets – four of which are in this star’s temperate habitable zone.
“Our analysis shows that even the closest stars generally spend more than 1,000 years at an observation point where they can see Earth transits,” Kaltenegger said. “Assuming the opposite is true, this provides a healthy timeline for nominal civilizations to identify Earth as an interesting planet.”
Estimates of the number of rocky planets in their star’s habitable zone depend on the planet’s radius and the size and type of the host star.
The new estimates put the number of planets per star within the habitable zone at 1.28 – or between one and two per system.
The experimental concept of a habitable zone boundary actually depends on the amount of sunlight that Venus and young Mars would have received early in the solar system.
Since we hadn’t identified habitable or even rocky worlds around all the nearby stars, the team decided to estimate how many there might be.
According to the researchers, the debate about the rate of occurrence of rocky planets surrounding other stars continues.
However, they wanted to estimate the number of pessimistic predictions that would be used in this way for 25% of all stars with at least one rocky world to create a rough estimate.
This resulted in 508 rocky worlds in the habitable zone, the complete sample of more than 2,000 star systems capable of seeing Earth within 100 light-years.
And this is the area where the radio waves will almost travel – so people in this world might hear test messages from Guglielmo Marconi on the outer edge.
Within this limit they decided there would be about 29 potentially habitable worlds, which “could” host an alien civilization capable of listening to our radio.
However, communication can be difficult because a civilization 82 light-years away can just hear the broadcasts from the start of World War II, and we won’t receive a response until 2101 if they send it today.
The James Webb Space Telescope – expected to launch later this year – is set to take a detailed look at the many transiting worlds to discern their atmosphere.
Doing so will allow astronomers to eventually search for signs of life, as we may be able to detect only telltale chemical signals generated by living things.
The Breakthrough Starshot initiative is an ambitious project in progress that looks to launch a nano-sized spacecraft toward the nearest exoplanet discovered around Proxima Centauri – 4.2 light-years away from us – and completely characterize this world.
The results were published in the journal . Nature.
Source: Daily Mail