The discovery of alien life would be revolutionary. But what if we uncovered it on two—or even seven—planets all orbiting the same star?
That’s the tantalizing possibility offered by the cosmic grouping called TRAPPIST-1, where seven Earth-size worlds circle a star roughly 39 light-years away. According to a new study, those planets are packed so tightly around their stellar host that the seeds of life could be hopping between them with ease.
The study, conducted by Manasvi Lingam and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics, is based on a theory known as panspermia, which in turn is based on the fact that planetary debris can be swapped between the worlds in our solar system. This is especially true for neighboring rocky planets—for instance, asteroid strikes have sent fragments of Mars crash-landing onto Earth.
Panspermia takes this a step further and suggests that life could catch a ride on that debris, hitchhiking from one planet to the next. It might sound wild, but recent research shows that some extreme forms of life can survive conditions akin to an interplanetary journey. Some scientists even argue that the seeds of life on Earth could have come from Mars.
In the TRAPPIST-1 system, all seven planets are nestled within a region that’s more than 20 times smaller than the distance between Mars and Earth. Such close proximity raises the tantalizing possibility that panspermia could take place in this system with ease.
Now, Lingam and Loeb have calculated that exact probability. Comparing the TRAPPIST-1 planets to Earth and Mars, they found that the travel time between one planet and the next is shorter by a factor of a hundred. This boosts the chance that life can survive such a harrowing journey. They also found that the likelihood of one planet’s debris landing on another is larger by a factor of 20 or so.
Altogether, the possibility that life can play hopscotch from one planet to the next is a few thousand times higher among the TRAPPIST-1 worlds than the possibility that it did the same from Mars to Earth.
“In a single planetary system, like TRAPPIST-1, the interchange of bacterial life is almost inevitable,” says the University of Buckingham’s Chandra Wickramasinghe.
Even better for the imagined aliens, all of the TRAPPIST-1 planets can potentially host life, given the right conditions.
Three of the worlds orbit in the star’s habitable zone, the region where they receive the perfect amount of heat for liquid water to flow on their surfaces. The rest of the worlds are temperate, meaning they might be equally warm if they have the right internal temperatures and atmospheric blankets.
“We might find forms of life that survive under conditions that we haven't anticipated,” says Loeb. “That's why it's exciting. We shouldn't have any prejudice, but should look at all seven planets in TRAPPIST-1.”
BLOWIN’ IN THE WIND
Of course, right now there’s no direct evidence that panspermia happens in our solar system or beyond. And some astronomers are doubtful that hitchhikers could survive such a traumatic journey.
First, the building blocks of life would have to endure extreme heat and pressure from the impact that spewed them into space. Out in the open void, they would be subjected to harsh ultraviolet radiation from their host star for potentially millions of years. Finally, they would once again face blazing temperatures as they fell from the sky and crash-landed in yet another violent impact.
“The poor organism would be fried twice and would be radiated by ultraviolet photons,” says Brice-Olivier Demory of the University of Bern, a co-author on the study that announced the TRAPPIST-1 discovery last month.
Amaury Triaud, a University of Cambridge astronomer who also co-discovered the TRAPPIST-1 planets, is on the fence: “I'm a skeptic about this,” he says. “But I also have to remind myself that life has managed to survive in extreme conditions.”
Bacteria have persevered inside nuclear reactors and on the outer rim of the International Space Station. Tardigrades—tiny water-dwelling invertebrates that look like chubby bears—have endured the vacuum of space for up to 10 days. And organisms frozen in Antarctic ice for centuries have been revived in labs. (Also see “Weird Life Found Trapped in Giant Underground Crystals.”)
Wickramasinghe also points out that not every cell flung between worlds needs to survive: “It's like throwing seeds in the wind,” he says. “Most are destined to be destroyed. But a very few could take fruit—and that's all that's needed.”
Already, the discovery team is planning to turn the Hubble Space Telescope toward the seven siblings, along with the next-generation James Webb Space Telescope, due to launch in 2018. If the TRAPPIST-1 planets have atmospheres, these powerful instruments will be able to discern some of the molecules in the air and potentially find the fingerprints of life.
Should astronomers find signs of life on one planet, that will motivate them to scour its neighbors for similar signatures—even if the search comes up empty-handed at first. And if they find a match, it will suggest that the planets are indeed swapping life.
“That’s a golden opportunity to study panspermia,” Loeb says.
And if they find the right evidence, that would imply life doesn’t necessarily have to start over again on each new planet. Instead, it might spread like wildfire throughout a planetary system—and maybe even the cosmos.
Follow Shannon Hall on Twitter.