Yet the Earth analogues—same-size rocky planets residing in the habitable zone of Sun-like stars—remain hidden. So far, astronomers haven’t confirmed any.
“None,” says Don Pollacco, professor of astrophysics at the University of Warwick. “No Earth analogues at all.”
But now a new robotic satellite expects to discover scores of them.
The European Space Agency’s PLATO probe might detect “a hundred, maybe a couple hundred analogues,” says Pollacco, the mission’s science coordinator.
That’s roughly one percent of the approximately 20,000 planets PLATO hopes to find: gas giants, icy worlds, places nothing like Earth.
“But the really important ones are the analogues,” says Pollacco.
Those planets, astronomers say, are probably the best prospects for extraterrestrial life. How many of them exist throughout the galaxy, no one knows.
But PLATO’s 200 or so sightings will likely be a trifle compared to what’s actually out there. Assume 100 billion planets are in the Milky Way (NASA’s estimate). Should analogues be one percent—that’s one billion Earth-like worlds.
Already, other space telescopes—like Kepler—have found a few Earth-like candidates. But they’re too far away to study. Researchers can’t get the data to verify details. Precise measurements of the planet’s size, temperature, density, and atmosphere are problematic.
That’s why PLATO will look for “nearby examples,” says Pollacco, worlds orbiting nearby stars “up to 100 light years away.” That’s about 600 trillion miles. In space, that’s close.
Close enough, at least, for follow-up observations from ground-based telescopes, including the VLT—Very Large Telescope array—in Chile’s Atacama Desert.
“That’s absolutely vital to the success of the mission,” he says.
By combining both space and ground data, researchers can ascertain if a newly-discovered world is an analogue or an also-ran.
“We’ll know their ages, their masses, their densities,” says Pollacco. Knowing density, in particular, is critical. High density planets are probably rocky and perhaps more Earth-like; low density suggests a gas or ice world.
PLATO—outfitted with 26 telescopes—will find the analogues by examining transits. Those are eclipse-like events, when a planet passes between its star and the spacecraft.
As that happens, the planet blocks a bit of the star’s light. The dimming, miniscule but measurable, indicates a world is there.
Ultimately, PLATO will produce an unprecedented “data base of planets,” Pollacco says. Potential analogues will get the most attention; determining what’s in their atmospheres is a priority.
One thing is certain. No longer is the search for other planets only a census. About 4,000 worlds outside our solar system have been confirmed. Discoveries are becoming routine. Now researchers want to know more, much more. “We’re in a new era,” says Pollacco.
But locating Earth-like worlds—and learning if life is on them—is a long haul. Assembling a spacecraft from drawing board to launching pad takes decades. PLATO, first proposed in 2007, lifts off in 2026. Total cost is about $1.1 billion (US dollars). More than 500 scientists in 20 countries are on the project.
It’s possible, says Pollacco, that none of the analogues discovered by PLATO will have life. Or that any life found will be simple, not smart. Just alien bacteria, not technologically-advanced beings.
“Who’s to know,” he says, “if the coincidences that happened on Earth happened anywhere else. We have to be really careful. We have an example of one.”
But then, speculating for a moment: “My feeling is that life is widespread in the universe.”
Including intelligent life?
“The scientist in me says, ‘We have no idea. We don’t know.’
“But in my heart, I want to say ‘yes.’”