Researchers believe it may be one of the best candidates for a closer look in the future by the James Webb Space Telescope, which NASA will launch in 2018.
LHS 1140b, named for the small, faint red dwarf star it orbits, has all the right features. It is rocky and right in the sweet spot of the habitable zone of its star, meaning it could support liquid water, and potentially life as we know it, on the surface.
It's designated as a super-Earth because the planet is 40% larger than our home planet -- 11,000 miles in diameter, 6.6 times the mass of Earth and a much higher density. They estimate that the planet is five billion years old. And it may even have an atmosphere.
Before the discovery of super-Earths, Earth was considered to be the largest rocky planet. The next largest planets in our solar system are gaseous, like Uranus and Neptune. Previous studies suggest that planets larger than 1.5 times the size of Earth are usually gaseous, said Jason Dittmann, lead study author and postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics.
Super-Earths exist in between. Several have been found in the habitable zone of their stars through NASA's Kepler
missions, although they are farther away and could later be proven to be gaseous.
"It's very interesting to me that we have just discovered a super-Earth right up against that boundary," Dittmann said. "The fact that the planet is rocky and in its star's habitable zone also raises its intrigue, because we may now have a planet suitable for the search for life as well."
Dittmann calls LHS 1140b the most exciting exoplanet he's seen in the past decade.
But for a planet that's orbiting a star 39 light-years away in the constellation of Cetus, how do we know such details? It all started with the observation of a dip in the light of the star.
How to find a super-Earth
In 2015, Dittmann was going over some observation data of small red stars collected by two robotically controlled observatories, called the MEarth Project, when he spotted something promising. The signal for star LHS 1140 looked like the telltale dip in light that signifies an exoplanet in orbit.
He approached the European Southern Observatory's HARPS team, home of the High Accuracy Radial velocity Planet Searcher, and they confirmed it. They were also able to help him discover its orbit, which is 25 days, as well as the planet's mass and density.
This also allowed them to be sure that the planet is rocky, said HARPS team member Nicola Astudillo-Defru, because HARPS is the most precise instrument that can measure the "wobble" of the planet as the star tugs on it. That is proportional to the planet's mass.
"The density that we measure shows that LHS 1140b is primarily composed of rocky material like iron and silicates, the same type of things that the Earth is composed of," Dittmann said.
In the future, they want to use HARPS to continue observing the star and see if there are more planets around it.
Why there's hope for habitability
LHS 1140b may seem like it's ideal now, but that wasn't always the case. The exoplanet had a "hellish" past.
When the star formed, it started out larger before condensing down to its dwarf star size. This process could take 40 million years, Dittmann said. During that time, the habitable zone of the star would be much further out, which means that LHS 1140b was exceptionally heated.
For that duration, it would be easy for the planet to lose all of its potential water for good. That's what happened to Venus
But if the planet can shield and hide its water source from this heat, it would be saved, Dittmann said. That's where lava comes in.
When planets first form, they're also hot, even molten, with the potential for giant magma oceans. Hot, molten lava can trap and hold onto water and other gases, Dittmann said.
Once the star contracted, the planet's magma would cool and release gases and water, creating a place more suitable for life right at the time that it moved into the habitable zone of the star. That steam would also help it create and maintain an atmosphere.
What is the range of the habitable zone? Think of it like holding an object too close to an open flame. Too close, and the object will burn. Too far away and the object won't be warmed. Just the right distance and equilibrium is achieved.
The habitable zone is also dependent on the type of star the planet orbits. LHS 1140b is 10 times closer to its star than we are to the sun, but it only receives half as much light because the star is so dim.
LHS 1140b's host star spins slower and emits less radiation, unlike the TRAPPIST-1 star. This affects the planet's ability to maintain an atmosphere, water and stable compounds.
The TRAPPIST-1 planets are also smaller and one has already been proven not to be rocky through a density measurement. But LHS 1140b's large mass could make the study of its potential atmosphere tricky.
LHS 1140b's star is comparatively larger than TRAPPIST-1 and brighter, so it could be an object of study for future ground-based telescopes in addition to the James Webb Space Telescope.
The James Webb Space Telescope is capable of observing large exoplanets and detecting starlight filtered through their atmosphere, which will enable scientists to determine the atmospheric composition and analyze them for gases that can create a biological ecosystem.
Dittmann thinks that the TRAPPIST-1 planets and LHS 1140b are exciting in their own right, and all deserve to be studied further.
"I hope that we can go after both of these systems' atmospheres so that we can compare them to each other," Dittmann said. "It's great that we have two systems, one in each flavor, so that we can actually see if this high-energy radiation actually does make a difference."
Julien de Wit, one of the researchers that discovered the TRAPPIST-1 planets, agrees. "We've been delighted to hear about the discovery of LHS by our colleagues from Harvard," he said. "In the search for signs of habitability, signs of life elsewhere, the more the merrier. LHS is one of these planets around small nearby red dwarfs, the ones we need to find as soon (and as many) as possible to study them with the JWST."
More observations using the Hubble Space Telescope are underway and the researchers are already using everything they can to study the planet and its atmosphere. They want to confirm the existence of the planet's atmosphere and find out if it has molecular oxygen or water, and if it's similar to Earth.
"What I truly find exciting is that we have a potentially habitable, rocky planet orbiting a nearby star that is currently very calm and stable and doesn't flare," Dittmann said. "We're essentially running down the list of the things we want in a habitable planet and checking the boxes."