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Exoplanet update: Discovery of seven Earth-like planets heats up search for life

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Seven newly discovered Earth-sized exoplanets may have the right size and location for life, say scientists – but could life survive their star's harsh ultraviolet radiation? 

An artist's conception of what the TRAPPIST-1 planetary system may look like, based on available data about their diameters, masses, and distances from the host star. Color and other details about appearance are completely speculative.
NASA/JPL-Caltech/AP
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Caption

“The universe is a pretty big place. If it's just us, seems like an awful waste of space,” Carl Sagan once said. We still don’t know if we’re alone or not, but a new discovery suggests that at least one nearby solar system makes good use of its space indeed.

Seven Earth-sized planets densely populate the area around a nearby dwarf star, circling it in tight, fast ellipses, announced an international team of scientists on Wednesday. An unprecedented three of those seven planets could support oceans, making them prime candidates in the search for life, and upcoming space telescopes promise to reveal more about the fascinating system in the near future – including how much potentially deadly radiation the star TRAPPIST-1 could be unleashing on its planets.

“This is an amazing planetary system – not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth!” lead author Michaël Gillon, of the STAR Institute at the University of Liège in Belgium, said in a press release. 

Sitting at a Millennium Falcon-friendly 12 parsecs (39 light years) away, ultracool dwarf star TRAPPIST-1 is relatively close to Earth, but don’t bother trying to find it in the sky tonight. It's just a little larger than Jupiter and burns about 2,000 times more dimly than our sun.

Despite its unassuming stature, this mini-star is home to seven planets, all about the same mass as Earth, give or take a third. They zoom around their host at dizzying rates, with orbits ranging from about two days to two weeks. If dropped into our solar system, the whole bunch would fit comfortably inside the orbit of Mercury.

An observer on any one planet’s surface would be treated to a view of several planets hanging in the sky, each looking larger than our moon appears to us, say scientists. Inter-planetary trips would take days, rather than months or years.

But what’s really turning heads is where the planets orbit relative to their host. Astronomers are especially interested in the area around a star where surface temperatures are not too hot and not too cold for liquid water to exist. Nicknamed “the Goldilocks zone,” this habitable band is just right for liquid water to support life as we know it.

The TRAPPIST-1 system is much more compact than our solar system, but because dwarf stars emit so much less energy than our sun, that turns out to be just right for three of the seven planets.

"What is significant about this system is the number of rocky, Earth-sized planets, and the number of planets in the habitable zone, both of which are unprecedented," Chris Copperwheat, one of the paper's co-authors and the head astronomer at the Astrophysics Research Institute of Liverpool, tells The Christian Science Monitor in an email.

In this respect, the newly discovered system may be even more habitable than our own.

"TRAPPIST-1 now holds the record for the most rocky planets in the habitable zone," says Lisa Kaltenegger, the director of the Carl Sagan Institute at Cornell University, who was not part of the study.

"Our solar system only has two (Earth and Mars)," she writes in an email to the Monitor. "We have other systems with up to seven planets, but we don't have a system with seven rocky ones."

Even the outliers could support at least some water, depending on the amount of heat produced internally by the gravitational stretching of the worlds, a process known as tidal heating.

A cosmic accident of geometry made the discovery possible. The solar system spins in such a way that, as viewed from Earth, the seven observed planets pass directly between TRAPPIST-1 and our telescopes. When these transits take place, the star dims just a little, its brightness dropping about 1 percent.

Gillon’s team had already known that TRAPPIST-1 was home to exoplanets, observing three crossing simultaneously in 2015. But uncovering the rest of the family was a team effort involving data from telescopes in Chile, Morocco, Hawaii, the Canary Islands, South Africa, and NASA’s Spitzer Space Telescope, which observed the system continuously for 20 days straight.

Now the question on everyone’s lips is, what about life? Scientists are a long way from answering the question conclusively, but excitement is high.

"Looking for life elsewhere, this system is probably our best bet as of today," co-author Brice-Olivier Demory, a professor at the University of Bern’s Center for Space and Habitability, said in a press release.

Dr. Copperwheat agrees that initial signs are promising, if scant.

"I think this is a very significant discovery – certainly one of the most exciting I have been involved with in my career," he says. "This is a very interesting and complex system which will be a key future target for the search for Earth-like conditions and life."

The most tantalizing targets are the three middle planets. In their paper, published in Nature on Wednesday, the researchers speculate that they might be home to a familiar feature: liquid-water oceans.

"Using a one-dimensional cloud-free climate model that accounts for the low-temperature spectrum of the host star, we deduce that planets e, f and g could harbor water oceans on their surfaces, assuming Earth-like atmospheres," they wrote.

In addition to their Goldilocks real estate, the planets are all less dense than the Earth, says Copperwheat, which implies dynamic compositions potentially featuring liquid water, plentiful ice, or extended atmospheres.

But everything hinges on that assumption of Earth-like atmospheres, which are far from a sure bet.

Remember that Mars falls in the sun’s habitable zone, too, but surface water doesn’t hang around too long, even on a nice day, before the ultra-thin atmosphere lets it boil off into space.

Just how life-friendly this kind of dwarf star might be is a hot topic, since the long-lived, slow-burning stars are paradoxically much more active than our sun, constantly shooting off solar flares that may bathe these super-close planets in high levels of harsh ultraviolet and X-ray radiation.

A recent paper from NASA considered just this effect, concluding that our neighboring dwarf star Proxima Centauri would likely erode any atmosphere that may exist around orbiting planet Proxima b over the course of about a hundred million years.

The same process could spell trouble for anything orbiting around TRAPPIST-1.

The dwarf star's X-ray emission is roughly the same as our sun's, says Copperwheat, but "these planets are a lot closer so will suffer a greater degree of irradiation."

That's not necessarily a deal-breaker for life, he cautions.

"The short answer is that we don't know what the long-term consequences of high-energy radiation are to the habitability of Earth-type planets," he writes.

"It may strip off the atmospheres, rendering the planets inhospitable to life, but on the other hand it could actually help by just stripping off the hydrogen and helium," he explains: atmospheric ingredients that, some scientists have argued, are not conducive to life.

Dr. Kaltenegger, currently in the process of publishing papers modeling atmospheric erosion of both Proxima b and the TRAPPIST-1 planets, sees plenty of potential even for environments bathed in UV radiation. She points out that planets in either system could keep their atmospheres if they have Earth-like features like a magnetic field or an ozone layer.

"I would not worry too much about a complete erosion of the atmosphere, but a thinner atmosphere is definitely possible, although that would still be able to shelter an ocean," she explains. "Life is a definite possibility on these worlds... but it might look different."

Specifically, it might glow.

Kaltenegger published a paper last summer outlining one UV survival strategy, based on Earth's bioluminescent corals. Organisms on planets around a dwarf star could protect themselves from the damaging rays by absorbing the UV radiation, and then releasing it at a longer, safer wavelength, she theorized.

Such an ecosystem could react to solar flares by literally lighting up the planet, a sign she proposes could be observed from Earth.

With so many theories flying around, astronomers’ next task is clear: Observe the TRAPPIST-1 system and gather as much data as possible to answer some of these questions.

“At the moment, theoretical work on these questions is I think somewhat inconclusive, so it's up to observers like myself to actually try and detect the atmospheres to better inform the models,” Copperwheat explains.

Fortunately, they might not have to wait long. A number of next-gen planet finders will come online next year, including the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope.

Scientists have high hopes in particular for the James Webb Space Telescope, which should be able to take direct measurements of the planets as they cross in front of TRAPPIST-1, revealing tell-tale signs of composition, atmosphere, and potential biosignatures like ozone.

“The James Webb Space Telescope, Hubble’s successor, will have the possibility to detect the signature of ozone if this molecule is present in the atmosphere of one of these planets,” explained Dr. Demory in a press release. “This could be an indicator for biological activity on the planet.”

And the signal shouldn’t be hard to pick up. Unlike our planet, which transits the sun only once every 365 days, the near-daily frequency with which these seven planets transit TRAPPIST-1 basically guarantees good chances for observation.

Kaltenegger says that finding biosignatures requires a clear view of the planet and “roughly 70 to 100 hours (of observation) as a rule of thumb.”

Copperwheat is also looking forward to the data collection bonanza to come, saying the system “is going to be intensively studied for many years to come” to help determine its habitability.

Even if all seven worlds turn out to be solar flare-roasted wastelands, Copperwheat suggests we’ve still learned an important lesson about our place in the cosmos: "It seems Earth-sized planets may be very common in the Universe!"