As finding alien worlds has gotten easier, learning every single detail scientists can has become, perhaps surprisingly, a bit of a waste of precious time of instruments and computers alike.
To date, scientists have discovered 4,104 confirmed exoplanets. But for every confirmed planet that astronomers nail down, there are handfuls of maybe-planets in the data, whispers in the data that might come from stars hiccuping or pairs of stars dancing or would-be stars that didn't quite make the cut. And scientists no longer have the resources to analyze every potential planet's identity crisis.
"It's gotten to the point that we have so many to choose from now — there's so many exciting candidates coming in that we actually don't have to look at every single one and confirm every single one," Jessie Christiansen, an astronomer at Caltech and NASA's Exoplanet Science Institute, told Space.com. "You really have to prioritize, you have to look at this list of planets that are coming out and say, 'OK, which one do we really think we're going to learn the most about?'"
Properly confirming an exoplanet is a laborious process that requires scientists to determine both the size and the mass of the object to rule out other phenomena masquerading as a planet. Those observations use instruments that are in high demand from scientists studying a host of phenomena.
And the confirmation process can be time-consuming. In particularly knotty situations, Christiansen said, it can stretch up to a year. "Some of these planet candidates really, if it gets its hooks in you, if it becomes your thing that you're trying to solve," she said, "you can sink all the time into these things."
But there are two different ways to learn about exoplanets. One approach zooms in on individual planets to learn as much as possible — whether it is rocky or gaseous, whether it has an atmosphere and what that atmosphere looks like, how it may have become the way it is. But these questions can only be answered about planets that orbit particularly bright stars; otherwise, scientists can't get enough data.
The second approach looks at the diversity of planets across the universe as a population. "The Kepler mission was interested in statistics," Christiansen said. "The point was to get thousands of planets to put in our buckets and say, 'OK, this one is the most common, this one is the next most common and that kind of thing.'"
Which is precisely what the Kepler Space Telescope did between 2009 and 2018 during its two distinct missions, called Kepler and K2, finding more than 2,500 confirmed exoplanets. That bounty of riches prompted a shift in scientists' mindsets, Christiansen said, as individual worlds became less unique.
"If it's the 80th hot Jupiter that's been found and we don't have any reason to believe it's going to be different from the 79 that came before it," she said, "are we really going to scrutinize it in the same way that we scrutinized the first 79?"
And so, as the Kepler discoveries piled up, scientists introduced a new technique of evaluating potential planets, called validation. With more easily acquired observations, astronomers run a statistical model evaluating the probability of non-planet explanations for the data they have acquired; below a certain cutoff, that's good enough for scientists focused on surveying populations of exoplanets.
"There was this revolution in the field in our thinking, which was, we don't actually have to confirm every single one, we can validate them," Christiansen said. "So you believe it's a planet, statistically, but you actually haven't measured the mass. That's kind of the cheap — I'm using air quotes — that's kind of a cheap way of confirming planets."
But even validating planets is now too expensive a process for exoplanet scientists to apply to every potential world. And the scarcity of planet-confirmation resources astronomers face is only going to become a starker problem, Christiansen said.
In April 2018, NASA launched its new planet-finder, the Transiting Exoplanet Survey Satellite, or TESS. Scientists expect to confirm about 16,000 planets spotted in its data — but that requires sniffing out somewhere in the vicinity of 100,000 to 300,000 candidate planets and evaluating each.
"Now I have to look at all these candidates and decide which ones I even want to confirm," Christiansen said, who said she's excited by the bounty of exoplanets, despite the strict prioritization it requires. "I've been hunting for planets since 2004 — 15 years — and plenty of scientists have been hunting for even longer. And this is the first time I've really just sat back and gone, 'Wow. It's not worth doing some of this, just in terms of the time.'"
And the embarrassment of riches will only continue, she said. NASA's next exoplanet-finder, the Wide Field Infrared Survey Telescope (WFIRST), may allow scientists to discover 100,000 confirmed exoplanets — which means even more hundreds of thousands of candidates to evaluate.
"Coming up with new statistical ways to deal with this I think will be even more important as a tool going forward," Christiansen said. "We have more planets than we have resources, but that's only going to get worse and much worse, like, exponentially for the next decade."
- 7 Ways to Discover Alien Planets
- Photographing an Exoplanet: How Hard Can it Be?
- The Strangest Alien Planets in Pictures