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New Model: Nearby Exoplanet TRAPPIST-1e May Be Just Right for Life — **A new model suggests that TRAPPIST-1e, which has liquid water on its surface, is the most likely in the seven-planet system to be habitable.**
**Credit: NASA/JPL-Caltech**

Tucked between a boiled-away desert and a giant snowball, an alien world called TRAPPIST-1e may be the only habitable planet in a newly discovered batch of seven, according to a new climate model.

When researchers announced the discovery of seven planets that closely orbit the cool red star TRAPPIST-1 in February, there was a rush to learn more about the little rocky worlds and find out if any might be habitable. One of the first scientists to model the worlds' potential climates in depth was Eric Wolf, a researcher at University of Colorado, Boulder. His model, along with others still to come, will paint a clearer picture of what the system's planets could actually be like.

Wolf modeled three planets around TRAPPIST-1 and tested different potential atmospheres to see whether any let liquid water exist on a planet's surface. He found that, in his simulation, only one of the planets listed as potentially habitable would be able to keep liquid water. [Exoplanet Tour: Meet the 7 Earth-Size Planets of TRAPPIST-1]

Snowballs and runaways

In particular, Wolf investigated planets d, e and f around TRAPPIST-1, which lies about 39 light-years from Earth. He found that planet d orbits too close to its star, building up a thick atmosphere of water vapor that heats the planet even further and boils the rest of the water off — a runaway greenhouse effect that guarantees the planet winds up too hot and dry to host life over time. Planet f, by contrast, is too far from its star, making it a snowball; any surface water would be frozen solid, and in the model, no combination of gases in the planet's atmosphere seemed to keep it warm enough, Wolf said.

"So that leaves us with planet e as the one real chance of having a temperate, habitable, Earth-like climate," Wolf told Space.com.

To reach that conclusion, Wolf used a modified atmospheric model originally calibrated to simulate Earth's climate. TRAPPIST-1's planets, though Earth-size, exist in very different conditions than Earth does — the seven planets huddle close to their ultracool red dwarf star, and have full orbits lasting between 1.5 and 20 days (compared with Earth's 365.26 days). Because they are so close to their star, the planets likely orbit with one side always facing inward. The star gives off different wavelengths of light than the sun does; TRAPPIST-1 is much redder, and emits longer wavelengths, including those in the near-infrared.

Wolf used the Janus supercomputer at the University of Colorado and the Hyak supercomputer at the University of Washington to make his calculations. His article has been accepted for publication in The Astrophysical Journal, and in the meantime, he posted the results on ArXiv.org.

A new model simulated three planets around TRAPPIST-1 — d, e and f — with atmospheres consisting of various gas quantities and compositions to see whether liquid water could persist on their surfaces. Here, the planets' surface temperatures, water contained in clouds, thermal energy released and reflected light are graphed for different potential atmospheres. Planet f is always frozen and planet d is too hot for water, but depending on the atmosphere planet e can be cold, temperate or hot and still retain surface water.
**Credit: Eric Wolf/UC Boulder**

For this analysis, Wolf assumed that water was freely available on the surface of the planets. He found that with an Earth-like atmosphere, about 20 percent of planet e's surface isn't icy — it would look like an eyeball, with a warm, melted spot facing the star — and that you could duplicate Earth's temperature by adding much more carbon dioxide to the atmosphere (or, various combinations of nitrogen and carbon dioxide could offer that insulation).

To compensate for planet f's distance from the star and make it warm enough for liquid water, he tried adding about 30 bars pressure of carbon dioxide — approximately 30 times the pressure of Earth's entire atmosphere at sea level. However, planet f would still be too cold to host life — so cold, in fact, that the gas would start condensing and fall out of the atmosphere.

The farther-out planets would fare even more poorly, freezing any available water and becoming snowballs as well. Planet d would be too hot to hold water regardless of its atmosphere's thickness and composition, according to the model, and so would the closer-in planets. Although the original paper announcing the system a month ago, published in the journal Nature, posited that planets e, f and g could host liquid oceans, Wolf's more complex model suggests that f and g would be too bitterly cold.

"The main take-home point is that TRAPPIST-[1]e is the real winner here," Wolf said. But he's eagerly awaiting more models; as with modeling climate change on Earth, it will take many different techniques to come to a consensus about which planets' atmospheres could allow for liquid water.

"We need to intercompare between numerous models to come to a consensus that our models are doing the right thing, and that we can get some convergence of our results to make judgments about these planets," he added.

There are many variables that affect a planet's environment, and various models handle those variables in different ways. Wolf's model folds in what effect different gases in the atmosphere would have, the cooling impact of clouds and the way radiation heats the planet, among many other considerations. It also simplifies the way the ocean works; in reality, ocean currents could reduce the sea-ice coverage of the planet by transporting heat from the warmer dayside to the colder nightside. Other climate models might make different choices for each of those calculations, and they're always improving as researchers learn more about how a planet's environment works. [TRAPPIST-1 is 'Most Incredible Star System to Date' - 5 Amazing Facts]

What to look for

For TRAPPIST-1d, e and f, Wolf modeled the temperature across the planets while trying out particular atmospheres. For planet e, he tested cold, temperate and hot scenarios. Then, he graphed the heat that would be released from each planet with a given atmosphere, as well as the brightness of light reflected off the surface of the planet in each part of the orbit. Researchers will be able to compare those models to real data as it comes in.

"If we can get these observations and compare them to our model results, we might be able to make some estimates of what the atmosphere of the planet is like," Wolf said. Making these models in advance helps researchers prepare for that data, and also suggests which might be most productive to investigate further, he added. In the future, he and his colleagues plan to simulate the spectra of light that would come back from the planets as well, letting researchers match future observations with particular atmospheres.

"It's a really nice paper," said Anthony Del Genio, a researcher at NASA's Goddard Institute for Space Studies who was not involved in the study. "To my knowledge, Eric [Wolf] is the first one to do this type of study for the TRAPPIST-1 system — certainly, he's the first one to do a 3D study."

Del Genio is referring to a 3D climate model, which portions off the planet into vertical columns and tracks how conditions change across the columns, whereas a one-dimensional model examines just one column of atmosphere and surface. 3D models can incorporate more nuance in natural phenomena, and can handle differences in the dayside and nightside of the planet, which is particularly important for orbs like the TRAPPIST-1 planets that always face the same side toward the star.

Del Genio is looking forward to what other models might reveal, particularly details about planets d and f. Various treatments may provide different results for the planets' potential habitability, he said. But for now, Wolf's study offers a good suggestion of what to look for as researchers turn more powerful telescopes on the star system.

"He's really laid the groundwork for characterization of this system, and I'd say, if you are an astronomer thinking of observing the TRAPPIST system to characterize the atmosphere, that as a result of this study, you know which planet you want to go after first," Del Genio told Space.com.

Email Sarah Lewin at slewin@space.com or follow her @SarahExplains. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

PASADENA, Calif. — NASA's veteran Cassini mission will officially kick off its farewell tour of the Saturn system on April 22 with a final close encounter of the ringed planet's largest moon, Titan. Five months later, on Sept. 15, Cassini will spectacularly burn up in Saturn's crushing atmosphere.

But before that happens, Cassini will dive through a 1,200-mile (1,930 kilometers) gap between the planet and its innermost ring to carry out science that is only possible now that the mission is running out of fuel.

"What a spectacular end to a spectacular mission," Jim Green, director of NASA's Planetary Science Division at the agency's headquarters in Washington, D.C., said via video link at a special news conference held here today (April 4) at NASA's Jet Propulsion Laboratory (JPL). [Cassini's 'Grand Finale' at Saturn: NASA's Plan in Pictures]

The "Grand Finale" will begin when Titan's gravity hurls the spacecraft close to Saturn's atmosphere, ending the probe's series of ring-grazing orbits and causing Cassini to dive through a gap in the rings.

Over the next 22 orbits between the planet and the innermost ring, Cassini will embark on a completely different phase of discovery. The spacecraft will carry out scientific measurements of the apparently empty environment between the planet's upper-atmosphere gases and the innermost edge of Saturn's D-ring.

"The Grand Finale is a brand-new mission," said Linda Spilker, Cassini project scientist at JPL. "We're going to a place that we've never been before … and I think some of the biggest discoveries may come from these final orbits."

**Artist’s illustration showing NASA’s Cassini spacecraft above Saturn's northern hemisphere prior to one of the probe’s 22 “Grand Finale” dives.**
**Credit: NASA/JPL-Caltech**

This unique trajectory will allow Cassini to make incredibly detailed maps of Saturn's magnetic and gravitational fields, helping scientists understand what structures lie beneath the planet's atmosphere and, potentially, revealing the mechanisms behind Saturn's mysterious spin. This is also the best opportunity yet to get a good look at the mysterious hexagon etched into the clouds around Saturn's north pole, mission team members said.

Since arriving in Saturn orbit in 2004, Cassini has steered clear of the planet's icy ring material, opting for a more distant (and safer) orbit where the gas giant's family of moons could be studied and Saturn could be observed from afar. But now that the mission is in its final months, more risks can be taken to carry out observations that scientists would have only dreamed of earlier in the mission.

During the daring ring dives, Cassini will be closer to the planet than ever before. The probe will use its mass spectrometer to "taste" the chemistry of the tenuous gases on the outermost edge of Saturn's atmosphere and return the most detailed observations ever obtained of Saturn's high-altitude clouds and ring material. [Latest Saturn Photos by Cassini]

Mission team members are taking pains to minimize the dangers associated with this privileged view.

"We don't know exactly where that D-ring ends … so, to be on the safe side, we want to protect the spacecraft's instrumentation from these particles," Spilker said. After all, Cassini will be on a "ballistic trajectory," traveling at a speed of 76,000 mph (122,000 km/h).

"It's going to be kind of risky," Green said. "The ring material actually falls into Saturn, and it won't take much to stop our spacecraft at the velocity it is flying."

This artist’s illustration shows NASA’s Cassini probe plunging into Saturn’s atmosphere on Sept. 15, 2017. The spacecraft will work to keep its antenna pointed at Earth while it sends its final data, including the composition of Saturn's upper atmosphere.
**Credit: NASA/JPL-Caltech**

Although models of Saturn's rings suggest the inner gap will be mainly empty, mission scientists said they assume some small particles will hit Cassini. So, as the spacecraft makes its ring-plane crossings, it will be oriented in such a way to maximize the shielding of delicate instrumentation behind Cassini's high-gain antenna.

"We expect a lot of impacts, but it will be like going through smoke … we believe the D-ring extends into Saturn," Cassini program manager Earl Maize, also of JPL, told Space.com. He added that mission team members are confident that Cassini will encounter only the smallest particles in the ring gap, and the probability of losing the spacecraft to an impact with something bigger is "less than 1 percent."

Cassini's Grand Finale is occurring this year because the spacecraft has been so active and so prolific, Maize added.

"Back in 2010, we decided that we'd use every last kilogram of propellant to explore the Saturn system as thoroughly as we could," he said. "We'll end up with no gas in the tank, but at the same time, this is a very serious constraint. Ultimately, Cassini's discoveries (caused) its demise."

But why crash Cassini into Saturn? NASA decided to incinerate the probe to ensure that it doesn't careen into, and potentially contaminate, Titan or the fellow Saturn moon Enceladus, both of which may be capable of supporting life. (Cassini was sterilized before its October 1997 launch, but some microbes from Earth may still survive aboard the probe, agency officials have said.)

The $3.2 billion Cassini-Huygens mission — a joint effort of NASA, the European Space Agency and the Italian Space Agency — has made a number of mind-blowing discoveries during its time in the Saturn system.

For example, Cassini spotted liquid-hydrocarbon seas on Titan, making the moon the only place beyond Earth known to harbor bodies of stable liquid on its surface. (And the mission's piggyback lander, called Huygens, touched down on Titan in January 2005, becoming the first probe ever to land on a body in the outer solar system.)

Cassini also discovered Enceladus' amazing water plumes, which in turn helped reveal that the satellite hosts an ocean of liquid water beneath its icy shell. And the probe has imaged Saturn's diverse family of moons in detail that was not possible before.

In addition, the mission has studied the dynamics of Saturn's rings extensively, revealing the intricate relationships among planet, rings and moons. Saturn's unique environment has acted like a natural laboratory, demonstrating how moons are formed and destroyed — findings that can be scaled up to better understand how planets formed around the sun and, perhaps, around other stars, scientists have said. In short, the data gathered by the spacecraft will keep researchers busy for decades to come, scientists said.

Cassini has been a decades-long odyssey for the scientists involved, and the probe's 3-minute final dive into Saturn's atmosphere on Sept. 15 will be a bittersweet moment, the researchers said.

"I think that once the signal is lost, it would mean the heartbeat of Cassini is gone," said Spilker. "I think there will be a tremendous cheer and applause for the completion of an absolutely incredible mission. Hugs, tears — the Kleenex box will be passed around — but we will rejoice at being part of such a wonderful mission."

Follow Ian @astroengine and www.astroengine.com. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com