So far, we know of about 2,000 planets outside the Solar System. Many of these are hot and large worlds that are likely uninhabitable to life as we know it.
But a select few appear to be rather Earth-like. That is, they are similar in size to our own world, they orbit in the habitable zone of their star where liquid water can form, and some orbit Sun-like stars.
At the moment, we haven't found a world that is identical to our own, but we are close. In the coming years, we will find more and more Earth-like planets, but for now, here’s a look at some of the worlds most like our home that we’ve found so far.
Gliese 581d was just the start of our Earth-like discoveries. Tyrogthekreeper.
One of the earliest systems deemed to possibly play host to habitable planets was the Gliese 581 system. This red dwarf star is located a “mere” 20.3 light-years from Earth, and is thought to have several planets in orbit. One of these, Gliese 581d, orbits within the habitable zone of the star, with a year lasting about 66.87 Earth days.
While the planet is thought to be almost seven times more massive than Earth and 2.2 times as big in size, possibly making it a mini-Neptune, its location in the habitable zone led some to suggest it could be Earth-like. If it was rocky, it could potentially be a “super-Earth,” possibly with liquid water on its surface.
The actual existence of the planet itself, however, has been called into question. First discovered in 2007 at the La Silla Observatory in Chile, it was later declared a “false detection” in 2014, before a study this year reasserted its existence.
Whether it does exist or not, Gliese 581d would be pushing the limits of what we could describe as “Earth-like,” although it was one of the first such intriguing worlds. There are other, better candidates though…
This was the first exoplanet found in a habitable zone by Kepler. NASA/Ames/JPL-Caltech.
Once the Kepler space telescope launched in 2009, the number of known exoplanets shot up rapidly – and continues to increase. One of the earliest Kepler planets found was Kepler-22b, which garnered significant attention when it was discovered in 2011.
The planet is 2.4 times the size of Earth, but its mass is not known, meaning scientists aren’t sure if it is rocky or gaseous in composition. However, it was the first planet discovered by Kepler to be in the habitable zone of a star – the Sun-like Kepler 22 – which meant that it could have liquid water on its surface, if it is rocky.
Located 600 light-years from Earth, the planet completes an orbit of its host star every 290 days, not wholly dissimilar to Earth’s orbit. While its potential for habitability remains questionable, its discovery led the way to further, more Earth-like worlds.
There's a chance this planet is covered in a global ocean. NASA Ames/JPL-Caltech/Tim Pyle.
The discovery of this exoplanet in April 2013, a super-Earth, was enough to spark the U.S. House of Representatives to formally discuss “Exoplanet Discoveries: Have We Found Other Earths?” as part of two subcommittees.
That was probably a bit pre-emptive, but the planet is hugely intriguing. At just 1.4 times the size of Earth, some models suggested that the planet not only had liquid water, but that it could actually be engulfed in a global ocean. This would admittedly bring complications to any life there – without dry land, it is difficult for complex life to evolve – but its presence, if confirmed, would be a huge discovery nonetheless.
Kepler-62f, roughly seven billion years old, orbits its host red dwarf star every 267 days at a distance similar to Venus around our Sun. As the star is a red dwarf, the habitable zone is closer in than that in our own Solar System, placing Kepler-62f right inside it. Another planet in the system, Kepler-62e, is 1.6 times the size of Earth and may also be habitable.
While it's pretty distant at 1,200 light-years away, it’s definitely a leading candidate for one of the most Earth-like planets we’ve found.
This remains the planet most similar to Earth in size that is known to be in a habitable zone. NASA Ames/SETI Institute/JPL-Caltech.
While Kepler was busy finding a number of planets, one thing that remained elusive was a planet similar in size to Earth in the habitable zone of a star. That was, until the announcement of Kepler-186f in April 2014.
“Earth 2.0,” “Another Earth,” and “Earth Twin” were just some of the headlines that heralded its discovery. At 1.1 times Earth’s mass, it remains the planet most similar to Earth in size found in the habitable zone of a star to date. While that is not in itself an indication of life on the planet, the discovery was hugely important – proving that the existence of Earth-sized planets in habitable zones was not just a fluke in our Solar System.
“The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth,” said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington, in the statement announcing the discovery. “Finding a habitable zone planet comparable to Earth in size is a major step forward,” Elisa Quintana, a research scientist at the SETI Institute at NASA's Ames Research Center and lead author of study that found the planet, added.
An Earth-sized planet in the middle of a habitable zone ticks two boxes to be deemed “Earth-like,” but the star it orbits is just half the mass of our Sun. That meant there was still room for improvement. And that improvement arrived just over a year later.
Our best bet for Earth 2.0 yet. NASA/JPL-Caltech/T. Pyle.
Most of the exoplanets we've found are much bigger than Earth. Some are smaller. Some are too hot to support liquid water. Some orbit stars different to the Sun. And so on.
But one fulfills pretty much all the requirements of being Earth-like. That planet is Kepler-452b, announced in July 2015. It is only slightly bigger at 1.6 times the size of Earth, and has an almost exactly Earth-like year at 384.84 days. It also orbits a very similar star to our Sun, just 4% more massive and 10% brighter.
One interesting difference between this and our world is that Kepler-452b is 1.5 billion years older. This means that if it was or still is Earth-like, it is likely providing a glimpse into what will become of our planet in the future, rather than being an analog for what it might be today.
“If Kepler 452b is indeed a rocky planet, its location vis-a-vis its star could mean that it is just entering a runaway greenhouse phase of its climate history,” Doug Caldwell, a SETI Institute scientist working on the Kepler mission, said in a statement. “Kepler 452b could be experiencing now what the Earth will undergo more than a billion years from now, as the Sun ages and grows brighter.”
At 1,400 light-years away, we won’t be going there any time soon – but for now, it’s the closest thing we’ve got to an Earth 2.0.
We still haven't quite found an exact twin of Earth. But don't fret; astronomers are just starting to go through the bulk of the Kepler data that should harbor the most Earth-like planets, owing to the nature of how the transit method finds planets. This involves looking at dips in a host star's brightness as a planet crosses in front of it from our viewpoint, but astronomers have to observe three orbits before they can be confirmed. There could be a cavalcade of more Earth-like planet discoveries on the horizon.
It's highly likely that there are thousands, millions or even billions of Earth-like worlds awaiting our discovery in the Milky Way. And we're edging ever close to finding the first that is pretty much exactly the same as our own.