The search for extraterrestrial life is easily the most profound question in modern astronomy, but it’s hampered by a lack of both technology and time.
Is life possible beyond the solar system? If we’re ever to find out, we must study and categorise the stars to answer this one, simple question: if we had a spaceship we could send to the nearest Earth-like planet, which one would we send it to?
When astronomers find exoplanets, they put them on a list marked “potentially habitable” or else use them as clues that habitable exoplanets may lurk in their star system. Most of them are exceptionally far away. So far we’ve found three close exoplanets that orbit within a star’s so-called “habitable zone” where liquid water could exist on its surface.
If astronomers had to choose a planet in another star system to send a spaceship, these three would be prime candidates:
- Proxima Centauri b: an exoplanet orbiting an M-type red dwarf star 4.24 light years away in the constellation of Centauri.
- Tau Ceti e: an exoplanet orbiting an M-type red dwarf star 11.9 light years away in the constellation of Cetus.
- Teegarden b: an exoplanet orbiting an M-type red dwarf star 12 light years away in the constellation of Aries.
Where will we most likely find others? Though the vast majority of star systems remain unexplored, we know of plenty that contain planets not in the star’s habitable zone. These star systems are surely the best places to look.
Cue the Habitable Exoplanet Hunting Project, a global attempt attempt to discover potentially habitable exoplanets within 100 light years, and involving over 25 observatories.
What is the Habitable Exoplanet Hunting Project?
It’s a network of amateur astronomy observatories around the globe—from the U.S. and Uzbekistan to South Africa and Australia—that is studying 10 stars within 100 light years for signs of new, as yet unfound exoplanets. All of the stars that will be studied already have confirmed transiting exoplanets outside the so-called “habitable zone”. “We’ve chosen observatories in deserts or high regions or mountains because weather is always the main problem with projects like this,” says Alberto Caballero, an amateur astronomer at The Exoplanets Channel and the coordinator of the Habitable Exoplanet Hunting Project. “But we will need to find more observatories in the southern hemisphere.”
Why the focus on G, K and M-type stars?
“They’re the stars that are most likely to host exoplanets with water on their surface because they don’t flare,” says Caballero. “If a star flares, it can damage the atmosphere of the exoplanets.”
G-type stars are like our Sun, but the Habitable Exoplanet Hunting Project only has one on its list; Pi Men in Mensa. “K-type stars are even better than G-type for finding potentially habitable exoplanets,” says Caballero. “K-type stars last a bit longer than the Sun and they emit less radiation as x-rays.” The most promising potentially habitable exoplanet we know of—Kepler 442b—orbits a K-type star. “It could be super-habitable, even more habitable than Earth,” says Caballero.
That’s why the search for “Earth 2.0” doesn’t mean we’re looking for a “Sol 2.0.”
Why ‘red dwarfs’ are so important
“Most of the potentially habitable exoplanets discovered so far have been around M-type stars—red dwarfs—which are impossible to see with the naked eye.” The smallest and coolest kind of star on the “main sequence”, red dwarf stars are by far the most common type of star in our region of the Milky Way.
Using the NASA exoplanet archive, Caballero and his colleagues made a list of stars that don’t flare, have confirmed exoplanets, but don’t have potential habitable exoplanets. The model, ideal discovery, for the project is an exoplanet called LHS 1140 b, a dense and rocky “super Earth” planet (almost seven times bigger than Earth) orbiting within the habitable zone of the red dwarf star LHS 1140 about 40 light years distant in the constellation of Cetus. It was discovered in 2017 by the MEarth Project.
How big is the habitable zone of a star?
The size of a star’s habitable zone depends on the star; the hotter it is, the further away the habitable zone begins, but the wider it could be. For an M-type star, its habitable zone is close to the star, but very narrow. Consequently, the Habitable Exoplanet Hunting Project has been careful to ignore stars that have confirmed Jupiter-sized gas giant exoplanets in their habitable zones. “It’s thought that if there’s a Jupiter-sized exoplanet within the habitable zone of a star it causes perturbations in their orbits and has tidal effects on other exoplanets nearby,” says Caballero. “F-type stars have huge habitable zones, so it might be possible to find exoplanets around them even if a Jupiter-sized exoplanet was also present, but in G type and K-type it’s not good.”
The most promising candidates
Though red dwarfs are common and the easiest to study, it’s still K-type stars that are astronomers’ favored hunting ground for exoplanets in the habitable zone.
“We’re trying to monitor the stars 24/7 for about two months, so it’s easier for us if we focus on M-type stars because any exoplanets would have really short orbital periods,” says Caballero. “But the most ideal ones are K-type stars.” That makes these three star systems the big targets:
- HD 189733 A: a K-type star 63 light years away in the constellation of Vulpecula
- GJ 9827: a K-type star 97 light years away in the constellation of Pisces
- HD 21749 (K): a K-type star 55 light years away in the constellation of Reticulum
Can any stars hosting exoplanets be seen with the naked eye?
M-type stars are way too small to see, but you can see Tau Ceti in the constellation Cetus with the naked eye (look due south from the northern hemisphere before midnight at this time of year and it’s roughly level with Orion’s Belt). Tau Ceti e, a “super Earth” exoplanet almost four times the mass of Earth, orbits a G-type star every 163 days. It was found using the radial velocity method, which is when astronomers measure the slight “wobble” a star displays when it’s got planets in orbit. It’s detectable in the color of light from the star.
What about TESS?
There’s already an all-sky search for habitable exoplanets in the form of TESS—the Transiting Exoplanet Survey Satellite—NASA’s orbiting space telescope, which has already found 29 confirmed exoplanets.
“TESS is doing the same thing the Kepler Space Telescope did, but it’s focusing on bright stars within 200 light years,” says Caballero. “So far it’s not detected any potentially inhabited planets, but it’s only just starting on the northern hemisphere.”
How do we find more potentially habitable exoplanets?
Although the Habitable Exoplanet Hunting Project will use optical telescopes for a few years, the long-term answer for exoplanet-hunters isn’t the “transit” method, but spectrometry—the study of biosignatures in light. “We need better instruments,” says says Caballero, name-checking the fact that astronomers are now beginning to the Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations (ESPRESSO), the most advanced spectrometer in the world. “It’s not found any potential habitable exoplanets yet, but I think it will be able to find Earth-like planets at nearby stars—it’s all about having better technology.”
Wishing you clear skies and wide eyes.