It goes without saying the most Earth-like planet we know of is Earth. Barring a scenario where many Earths exist within a hypothetical multiverse, this is the only one we’ve got.
The qualities that make our planet Earth-like — its rockiness and mass among others — are important to researchers searching for other worlds like ours. Other galaxies could be full of sibling Earths: In fact, evidence suggests there may be as many as one Earth-like planet for every five Sun-like stars in the Milky Way alone.
Earth is our home, which makes it special in its own right. But there could be countless more like it waiting to be found or even born. The ongoing hunt for Earth-like worlds, especially over the next few decades, hopes to illuminate how ordinary and extraordinary our planet may be.
What is an Earth-like world?
Though there are some basic requirements for a world to be considered Earth-like, overall, it’s difficult to narrow down a definition.
We know that mass is key when it comes to identifying Earth-like worlds: Some research suggests that objects just 1.6 times the size of Earth are more likely to be gaseous, making them less similar to our planet.
“Usually we categorize planets by how massive they are,” Kaitlin Rasmussen, an astrophysicist at the University of Michigan, told The Planetary Society. “There’s no category below Earth, so even if there’s something Mercury-size, we would still call that an ‘Earth.’”
Mass is critical to spotting worlds like ours, but there are other factors to consider, such as a star’s habitable zone: the not-too-hot, not-too-cold region where liquid water can exist on a planet’s surface.. That’s where things get a little more nuanced.
“I think it’s a spectrum,” said astrophysicist Moiya McTier, who completed her doctoral thesis in exoplanet habitability. “There are points you can pass to make a planet more Earth-like, and I’d say mass or radius are some of the benchmarks. Being in the habitable zone of a star is also important. The world should be able to host water and have an atmosphere — not as thin as Mars, but not a puffy, Neptune-like atmosphere.”
Even with those guidelines, there’s a gray area. For example, some researchers would argue that a planet can’t be Earth-like unless it orbits around a sun similar to ours, that is, a G-type main sequence star.
Unpacking what it truly means to be “Earth-like” evokes a host of scientific and philosophical questions. Although defining that term is pretty complicated, there are some worlds that appear to fit the description, or at the very least, parts of it.
Earth-like worlds of interest
It’s impressive how much scientists have been able to uncover about these distant worlds in such a short time. With the help of ground and space-based telescopes, researchers have found about 4,000 confirmed exoplanets and thousands of other candidates. A relatively small number of these worlds bear similarities to Earth.
Some of them are:
In 2014, NASA’s Kepler Space Telescope discovered the first Earth-size world in the habitable zone of another star. The planet, dubbed Kepler-186f, is located about 500 light-years from Earth in the constellation Cygnus.
Kepler-186f is a noteworthy location in the search for life. McTier has previously studied the eccentricity of Kepler-186f’s orbit in order to learn about the planet’s potential habitability.
“Even if it spends part of its year in the habitable zone, if Kepler-186f’s orbit is too eccentric, it would have large temperature swings,” McTier said. “But I found in my research that Kepler-186f’s eccentricity is pretty close to zero. It doesn’t experience big temperature swings and so it’s more likely to be habitable.”
NASA considers exoplanet Kepler-452b and its star to be the closest analog to our planet and Sun so far. Though it’s 60% larger than Earth in diameter, Kepler-452b is thought to be rocky and within the habitable zone of a G-type star similar to ours.
Located about 40 light-years away, the TRAPPIST-1 system has quickly — and rightfully — garnered a lot of interest. Not only do all seven planets in the system appear to be Earth-size and rocky, three of them are located in the habitable zone of their star. While liquid water could theoretically pool on TRAPPIST-1e, f and g, the other planets are probably too close or far from their sun to be in this “goldilocks zone.”
While TRAPPIST-1 hosts some interesting prospects, its planets are not Earth’s long-lost twins. Some research based on computer modeling suggests even the habitable TRAPPIST-1 planets may have developed like Venus, making them too hot to host water. TRAPPIST-1e may be the only planet in the system still hospitable to life, but without more data, it’s impossible to confirm.
Since 2009, Planetary Society members and supporters have been helping Yale University astronomer Debra Fischer and other scientists improve exoplanet-hunting technologies. Learn more about Fischer's quest to find 100 Earth-like worlds.
The future of Earth-like world searching
To better predict the habitability of Earth-like worlds, exoplanet researchers need to understand the atmospheres around them. That takes some seriously powerful tools, and thankfully, they’re coming soon.
“In 10 years, we’re going to have ridiculously good telescopes,” Rasmussen said. “They’re going to be like, 30 meters across, 40 meters across, whereas the largest we have right now is about 10 meters across.” NASA’s Roman Space Telescope, set to launch sometime in the mid-2020s, is one such telescope: It’ll embark on a five-year journey to identify 2,500 new exoplanets.
Launching even sooner than that is NASA’s James Webb Space Telescope (JWST), which is poised to be one of the most powerful exoplanet hunters ever. A significant, explicit part of its mission is to study the atmosphere around Earth-like worlds. Plus, it’ll search for biosignatures — gases associated with past or present life.
There’s something deeply humbling in looking to far-flung worlds for something shared with our own. It takes a lot of precise circumstances for a planet like ours to exist. The idea that this exactness could repeat itself doesn’t subtract from how special Earth is; it amplifies the possibility that life can thrive somewhere else. It’s an honor to be this common.