HAVE WE REACHED PEAK EXOPLANET?
We’ve Found 3,700 Planets—But No Earth 2.0 Yet
‘We know there are thousands of planets out there. We don’t know a lot about them.’
As of Jan. 1, we’ve found more than 3,700 planets; about 2,500 of those were discovered by NASA’s Kepler satellite missions. We’ve also found 2,794 planetary systems and 622 “multiple planet systems,” according to the Extra-solar Planets Catalog, which tracks humankind’s hunt for planets.
Despite the thousands of planets we’ve found, though, we don’t know a lot. We know sizes, masses… and that’s about it.
“If you take a step back and ask what do we know, we know there are thousands of planets out there,” Sara Seager, an MIT professor who’s one of the lead exoplanet researchers in the world, told The Daily Beast. “We don’t know a lot about them.”
At this point, those planets aren’t much more than a few points of numerical data. But soon we might know more, as astronomers prioritize quality over quantity of exoplanet discoveries and dive into their compositions in our hunt for the next Earth.
In 2016, astronomers announced the discovery of Proxima Centauri b, heralded as the closest we’d come to finding an Earth twin: It was found orbiting the closest star to Earth, was just a little more massive than Earth, and appeared to be in the habitable “Goldilocks zone” around its star where—if everything went right—it could hold liquid water in some areas and maybe, just maybe, life.
But Proxima b wasn’t found in a vast, census-like headcount like Kepler. It wasn’t discovered accidentally or as one small part of a larger campaign. It was discovered by focusing a team of researchers calling themselves Pale Red Dot on one star over a near-half-decade, looking and waiting for the star to move in just the right way that suggests a planet is ever-so-slightly tugging on it. It was only through those laborious hours of research and poring over the faint star that the planet emerged from the data.
“It was almost—but not quite—unheard of, but not a popular idea, to put all your eggs in one basket and that paid off, and I think we’re going to see more of that,” Seager said.
The planet, however, was later found to likely emit radiation to suck out life-necessary oxygen, according to a paper released in February 2017 in The Astrophysical Journal Letters. In fact, Proxima b illustrates the problem of why the quest of finding the next Earth is so hard: We don’t know a lot about what these planets actually are.
The next frontier of telescopes
Seager pointed out the example of the James Webb Space Telescope (JWST), the Hubble successor NASA is sending out to the Moon next year. The tennis court-sized telescope will be tasked with imaging some of these exo-worlds and finding atmospheres around Earth-sized worlds, finding out if they’re truly habitable.
Technology has been a limiting factor in finding individual planets. One problem is that planets don’t emit light—they only reflect it. The few planets that do reflect enough light to be seen from on-or-near Earth are large, young, and very hot. Next generation telescopes like JWST will be able to finally get that down to (roughly) Earth-size, even if those planets will appear as a pixel or two.
Demand for JWST will be great, as gathering enough light to see a dim planet is a large undertaking. With time being split between exoplanets, distant galaxies, weird stars, and other demands of an orbital observatory, astronomers will have to be smart about what planets they go after, which will require finding the best places to do a star-by-star analysis.
But it’s got drawbacks. “Since direct imaging of these small worlds takes a long time, it is unlikely that JWST will observe a large sample of planets,” Fabo Feng, an exoplanet researcher at the University of Hertfordshire, said.
The first exoplanets were found by measuring how a planet tugged on a star in a process called radial velocity, and it only worked for very large planets. Kepler employed the transit method, which waits for a planet to pass in front of its star and blot the light just a bit. But not every planet transits, and radial velocity is only now getting to the point that it can find planets around the mass of Earth. (There are, of course, several other exoplanet hunting techniques, but they’ve yielded fewer results.)
Feng’s work on the Tau Ceti system last year stretched radial velocity to its absolute limits in order to find four planets around the nearby star. Feng had to build algorithms to find these four worlds—all between the mass of Earth and Neptune—based on weak “tug” signals that might otherwise have been written off as some kind of stellar activity. That study was the culmination of 10 years of work and more than 5,000 observations.
NASA plans instead to rely on information from the Transiting Exoplanet Survey Satellite, set for launch in March, to home in on the best worlds for JWST to study. It’ll look for the dimming caused by a planet passing in front of its star, just like Kepler. But unlike Kepler, it will specifically look for planets around bright stars ideal for study by the Webb telescope, ending up with a lower overall yield of exoplanets.
One of JWST’s first targets will likely be the TRAPPIST-1 star, a system of seven Earth-sized planets, some which may have the right conditions for liquid water. That planet is the end result of another survey that only looked at 50 stars. Michael Gillon, a project lead on the TRAPPIST telescope, said that TRAPPIST-1 was the only planet-bearing star found in four years of research. A recent extension of the mission added 50 more targets—and still only expects to yield a very small number of planets due to the low occurrence of transits.
“We don’t really expect [TRAPPIST] to find more planets, even if all ultracool stars have a planetary system similar to TRAPPIST-1,” Gillon said. Ultracool dwarfs are the smallest normal stars that have low temperatures, thus offering more temperate planets, referring to the temperature of the star and therefore its ability to potentially be near an Earth-like planet.
Next year, Gillon’s TRAPPIST follow-up, SPECULOOS (the Search for Habitable Planets Eclipsing Ultra-Cool Stars), will debut. TRAPPIST was meant as a prototype to find planets around small, “cool” stars. But if TRAPPIST was meant to find just a couple worlds, SPECULOOS—with a telescope in both hemispheres—is expected to find a few dozen at a rate of one or two per year.
Will we find another Earth?
Red Dots, the group formerly known as Pale Red Dots, recently widened their scope to Barnard’s Star and Ross 154, two stars near Proxima b. “There’s this thought now that the closest stars are of incredible value, and that it’s worth focusing on them especially now that we’ve shown that every star has something to show for itself,” Seager, who is not involved in the project, said.
But Red Dots is just one of a narrow searches out there. Project Blue aims to find a habitable planet around one of the stars in the binary pair of stars in Alpha Centauri; Proxima orbits both stars a significant distance away. There’s Seager’s own ASTERIA project, which launched a small satellite last year to figure out if you can send out mini-satellites to look for exoplanets on a star-by-star basis.
The hunt for the next Earth by surveying stars won’t go away by any means. But in order to find out anything about these planets and figure out if the smaller ones we see aren’t just Earth-size or Earth-mass, but truly Earth-like, will take a lot of work.
“Let’s say there’s an alien civilization not too far away looking at our solar system with the same tools we have, but maybe a little bit better,” Seager analogized. “They would see Venus and Earth, and they would be the same planet. They wouldn’t know if one planet was habitable or not. They’d probably be pretty excited, because they have two planets, possibly in the habitable zone, but they’re basically the same size and mass to within a few percent.”