Its been a very exciting time for us space scientists! This time, unsurprisingly, it’s all about my favourite thing! Exoplanets!
Spitzer Telescope Finds A Unique Solar System With Seven Potentially Habitable Planets
NASA’s Spitzer Space Telescope has revealed the prescence of these worlds. Three of these planets are firmly located in the habitable zone, the area around the parent star where a rocky planet is most likely to have liquid water. The discovery sets a new record for greatest number of habitable-zone planets found around a single star outside our solar system. All of these seven planets could have liquid water—key to life as we know it—under the right atmospheric conditions, but the chances are highest with the three in the habitable zone.
“This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life,” said Thomas Zurbuchen, associate administrator of the agency’s Science Mission Directorate in Washington. “Answering the question ‘are we alone’ is a top science priority and finding so many planets like these for the first time in the habitable zone is a remarkable step forward toward that goal.”
At about 40 light-years from Earth, the system of planets is relatively close to us, in the constellation Aquarius. This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. In May 2016, researchers using TRAPPIST announced they had discovered three planets in the system. Assisted by several ground-based telescopes, including the European Southern Observatory’s Very Large Telescope, Spitzer confirmed the existence of two of these planets and discovered five additional ones, increasing the number of known planets in the system to seven.
The new results were published Wednesday in the journal Nature, and announced at a news briefing at NASA Headquarters in Washington. Using Spitzer data, the team precisely measured the sizes of the seven planets and developed first estimates of the masses of six of them, allowing their density to be estimated. Based on their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further observations will not only help determine whether they are rich in water, but also possibly reveal whether any could have liquid water on their surfaces. The mass of the seventh and farthest exoplanet has not yet been estimated – scientists believe it could be an icy, “snowball-like” world, but further observations are needed.
“The seven wonders of TRAPPIST-1 are the first Earth-size planets that have been found orbiting this kind of star,” said Michael Gillon, lead author of the paper and the principal investigator of the TRAPPIST exoplanet survey at the University of Liege, Belgium. “It is also the best target yet for studying the atmospheres of potentially habitable, Earth-size worlds.”
Video Credit :JPL/NASA
In contrast to our sun, the TRAPPIST-1 star – classified as an ultra-cool dwarf – is so cool that liquid water could survive on planets orbiting very close to it, closer than is possible on planets in our solar system. Its also very small, not much bigger than Jupiter, so its almost verging on being a brown dwarf. In fact it very much reminds me of Arthur C. Clarke’s story 2010 when Jupiter does in fact become a star after a mass increase by those pesky monoliths!
All seven of the TRAPPIST-1 planetary orbits are closer to their host star than Mercury is to our sun. The planets also are very close to each other. If a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth’s sky. The planets may also be tidally locked to their star, which means the same side of the planet is always facing the star, therefore each side is either perpetual day or night. This could mean they have weather patterns totally unlike those on Earth, such as strong winds blowing from the day side to the night side, and extreme temperature changes.
Spitzer, an infrared telescope that trails Earth as it orbits the sun, was well-suited for studying TRAPPIST-1 because the star glows brightest in infrared light, whose wavelengths are longer than the eye can see. In the autumn of 2016, Spitzer observed TRAPPIST-1 nearly continuously for 500 hours. Spitzer is uniquely positioned in its orbit to observe enough transits of the planets in front of the host star to reveal the complex architecture of the system. Engineers optimized Spitzer’s ability to observe transiting planets during Spitzer’s “warm mission,” which began after the spacecraft’s coolant ran out as planned after the first five years of operations.
“This is the most exciting result I have seen in the 14 years of Spitzer operations,” said Sean Carey, manager of NASA’s Spitzer Science Center at Caltech/IPAC in Pasadena, California. “Spitzer will follow up in the fall to further refine our understanding of these planets so that the James Webb Space Telescope can follow up. More observations of the system are sure to reveal more secrets.”
Following up on the Spitzer discovery, NASA’s Hubble Space Telescope has initiated the screening of four of the planets, including the three inside the habitable zone. These observations aim at assessing the presence of puffy, hydrogen-dominated atmospheres, typical for gaseous worlds like Neptune, around these planets. In May 2016, the Hubble team observed the two innermost planets, and found no evidence for such puffy atmospheres. This strengthened the case that the planets closest to the star are rocky in nature.
“The TRAPPIST-1 system provides one of the best opportunities in the next decade to study the atmospheres around Earth-size planets,” said Nikole Lewis, co-leader of the Hubble study and astronomer at the Space Telescope Science Institute in Baltimore, Maryland. NASA’s planet-hunting Kepler space telescope also is studying the TRAPPIST-1 system, making measurements of the star’s minuscule changes in brightness due to transiting planets. Operating as the K2 mission, the spacecraft’s observations will allow astronomers to refine the properties of the known planets, as well as search for additional planets in the system. The K2 observations conclude in early March and will be made available on the public archive.
Spitzer, Hubble, and Kepler will help astronomers plan for follow-up studies using NASA’s upcoming James Webb Space Telescope, launching in 2018. With much greater sensitivity, Webb will be able to detect the chemical fingerprints of water, methane, oxygen, ozone, and other components of a planet’s atmosphere. Webb also will analyze planets’ temperatures and surface pressures, all key factors in assessing their habitability. Those chemicals are important biomarkers, paricularly ozone as it is largely dependent on life in its formation, as well as a very handy radiation umbrella. To date though, the biggest red flag would be CFC’s as they, as far as we know, cannot be made naturally, but as this system is so young, I for one would be truly amazed if they were found.
Can These Worlds Support Life?
This find of Earth-size planets huddled around an ultra-cool, red dwarf star could be little more than chunks of rock blasted by radiation, or cloud-covered worlds as lead meltingly hot as Venus. Or they could harbor exotic life-forms, thriving under skies of ruddy twilight. Scientists are pondering the possibilities after this week’s announcement: the discovery of seven worlds orbiting a small, cool star some 40 light-years away, all of them in the ballpark of our home planet in terms of their heft (mass) and size (diameter). Three of the planets reside in the “habitable zone” around their star, TRAPPIST-1, where calculations suggest that conditions might be right for liquid water to exist on their surfaces, though follow-up observations are needed to be sure. All seven are early ambassadors of a new generation of planet hunting targets.
Red dwarf stars, also called “M-dwarfs”, outnumber others, including yellow stars like our sun, by a factor of three to one, comprising nearly 75 percent of the stars in our galaxy. They also last far longer. Our Sun is a middle aged star, halfway through its lifetime on the main sequence so will probably become a red giant in about 5 billion years. This red dwarf is very likely to last trillions of years, quietly doing its thing. And their planets are proportionally larger compared to the small stars they orbit. That means small, rocky worlds orbiting the nearest red dwarfs will be primary targets for new, powerful telescopes coming online in the years ahead, both in space and on the ground as they are earier to find using the transit method.
“The majority of stars are M-dwarfs, which are faint and small and not very luminous,” said Martin Still, program scientist at NASA headquarters in Washington. “So the majority of places where you would look for planets are around these cool, small stars. We are interested in the nearest stars, and the nearest stars are mostly M-dwarfs.”
But these are sure to be perplexing planets, with strange properties that must be teased out by careful observation as well as computer simulations. Finding out whether they can support some form of life, and what kind, likely will keep astrobiologists working overtime, perhaps attempting to recreate in the laboratory some of the conditions on these red-tinged worlds. Did I ever mention I love what I do?
“We’re definitely all working overtime now,” said Nancy Kiang, an astrobiologist at NASA’s Goddard Institute for Space Studies in New York City.
Expert opinion on whether red dwarf planets are suitable for life tends to tick back and forth, “like a pendulum,” said Shawn Domagal-Goldman, a research space scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“We’ve come up with these theoretical reasons why such a planet might struggle to be habitable,” he said. “Then we look at those theoretical concerns with a little bit more detail, and find out it’s not that big of a concern. Then some other theoretical concern crops up.”
At the moment, the pendulum is ticking back toward lifelessness. Recent findings suggest life would have an uphill battle on a planet close to a red dwarf, largely because such stars are extremely active in their early years, shooting off potentially lethal flares and bursts of radiation much like our nearest neighbour with planets Proxima b whis is thought to be a flare star. These youthful tantrums would go on for quite some time. The exact age of the TRAPPIST-1 star is unknown, but scientists believe it is at least 500 million years old, or about one-tenth the age of our 4.5-billion-year-old sun.
Image Credit: Newsapi.com.au
Red dwarfs could take their first billion years just to calm down enough to allow any nearby planets to be habitable. And the habitable zone around such stars is very close indeed. All seven of the Earth-size planets crowd so close to their star that they complete a single orbit of their year in a matter of days, 1.5 days for the nearest planet and 20 days for the furthest. That kind of proximity means the planets are probably tidally locked, with one face always turned to the star, the same way our moon presents only one face to Earth. And while red dwarfs are cool compared to our sun, they would loom large in the sky of a close, tidally locked planet, perhaps baking the sunward face. The far side, meanwhile, could be trapped in an eternal, frozen night.
The right kind of atmosphere could mitigate such effects, transporting heat to the planet’s far side and helping to moderate the climate overall. A recent study that relied on computer simulations of red dwarf planets, however, delivered more grim news. The flaring tempers of young red dwarfs, with their bursts of high-energy X-rays and ultraviolet emissions, could actually strip oxygen from the atmospheres of nearby planets, according to the study by a team at NASA Goddard led by Vladimir Airapetian. Other scenarios involve stripping away the atmosphere altogether. Yet another potentially sterilizing effect, even for M-dwarf planets that manage to hold on to their atmospheres, would result from high-energy radiation triggering a runaway greenhouse effect, Domagal-Goldman said.
“Maybe you would end up in a stable climate that’s too hot to support life,” he said.
But so little is known about how life gets its start, and how common or rare it might be in the cosmos, that tenacious life on M-dwarf planets remains a distinct possibility. Although loss of atmosphere from early stellar flaring is a legitimate concern, it is based on complex computer modeling, said Franck Selsis of the University of Bordeaux, one of the authors of the TRAPPIST-1 paper. Since computer models contain certain assumptions about stars and planets, they may not be complete, Selsis wrote in an e-mail. Models might fail to account for effects from the star on planetary atmospheres that could create a protective magnetic field. Or they might produce atmospheric loss rates so high they are physically implausible.
As for TRAPPIST-1, “The current relative quietness of the star and plausible sources of atmospheric replenishment still make possible for the planets to have atmospheres and surface habitable conditions,” said Michaël Gillon, principal investigator of TRAPPIST at the University of Liège, Belgium. “Our only way to go beyond these theoretical speculations is to try detecting and studying thoroughly their atmospheres.”
Other scientists also offered possible scenarios on the optimistic side of the M-dwarf habitability equation.
“Maybe the atmosphere can recover, and it’s just fine,” said Tom Barclay, a senior research scientist at the NASA Ames Research Center in Moffett Field, California. Barclay worked on the most prolific planet-finder, NASA’s Kepler space telescope, during both its original mission and its second incarnation, known as K2.
In Barclay’s scenario, lifeforms find a way to adapt to bursts of stellar radiation.
“You have regular events, but life is used to this,” Barclay said. “It just deals with it. We certainly see life on Earth capable of hibernating for very extended periods of time. We see that life goes into a state where it shuts down, sometimes for years or decades. So I think we shouldn’t, probably, rule it out, but we should put a lot of effort into studying whether this is a place where we think life could thrive.”
Future telescopes, including NASA’s James Webb Space Telescope (JWST), to be launched in 2018, could help resolve such questions by closely analyzing the atmospheric gases of the TRAPPIST-1 planets. If one of these instruments were to discover water vapor and, say, a combination of oxygen and methane, it could be a strong indication of a potential life-bearing world. The Hubble Space Telescope also will be a key player in characterizing the atmospheres of the TRAPPIST-1 planets and has, in fact, already begun a preliminary survey. Both space telescopes are equipped to capture the spectrum of light from the planets, revealing the types of gases that are present.
“We will look at atmospheres effectively in different wavelengths, allowing us to get the composition, temperature, pressure,” said Julien de Wit, a postdoctoral researcher at the Massachusetts Institute of Technology, Cambridge, and an author of the new TRAPPIST-1 paper. “This will allow us to constrain habitability.”
Besides, stellar flaring might not be all bad, that is, if M-dwarf planets have a bit of well-timed luck.
“They might start out with dense hydrogen envelopes that get blasted off,” said Victoria Meadows of the University of Washington, the principal investigator for the NASA Astrobiology Institute’s Virtual Planetary Laboratory. “So it’s kind of like a protective skin on the planet.”
The stellar radiation would remove the hydrogen, leaving a potentially habitable world behind. The planets also might form farther away from the star, moving closer over time.
“They could migrate in from outside, further out in the planetary system, where there is more water, cooler temperatures,” she said. “That would give them more protection against water loss. There are a whole bunch of options.”
Modeling shows, in fact, that densely packed M-dwarf planetary systems similar to the TRAPPIST-1 system, are more likely to form farther away, then migrate inward, because the inner solar system would lack enough material to form so many planets. In any case, if such planets possess life at all, simple lifeforms appear to be more likely. There are intruiging ways by which life could adapt, such as bioflourescence, which is fascinating enough by itself!
“I’m just talking about slime here,” Meadows said. “It’s far easier to evolve than sentient beings. The majority of life we find out there is likely to be single cell, relatively primitive life. That’s the sort of thing we’d be looking for on planets orbiting these M-dwarfs.”
Naturally more research needs to be done, particularly with the eagerly awaited James Webb Space Telescope (JWST).
“If these planets have atmospheres, the James Webb Space Telescope will be the key to unlocking their secrets,” said Doug Hudgins, Exoplanet Program Scientist at NASA Headquarters in Washington. “In the meantime, NASA’s missions like Spitzer, Hubble, and Kepler are following up on these planets.”
“These are the best Earth-sized planets for the James Webb Space Telescope to characterize, perhaps for its whole lifetime,” said Hannah Wakeford, postdoctoral fellow at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. At Goddard, engineers and scientists are currently testing the Webb telescope which will be able to view these planets in the infrared, beyond the capabilities we currently have. “The Webb telescope will increase the information we have about these planets immensely. With the extended wavelength coverage we will be able to see if their atmospheres have water, methane, carbon monoxide/dioxide and/or oxygen.”
When hunting for a potentially life-supporting planet, you need to know more than just the planet’s size or distance from its star. Detecting the relative proportions of these molecules in a planet’s atmosphere could tell researchers whether a planet could support life.
“For thousands of years, people have wondered, are there other planets like Earth out there? Do any support life?” said Sara Seager, astrophysicst and planetary scientist at MIT. “Now we have a bunch of planets that are accessible for further study to try to start to answer these ancient questions.” The number of planets in the system will also enable new research in the field of comparative planetology, which uncovers fundamental planetary processes by comparing different worlds. “This is the first and only system to have seven earth-sized planets, where three are in the habitable zone of the star,” said Wakeford. “It is also the first system bright enough, and small enough, to make it possible for us to look at each of these planets’ atmospheres. The more we can learn about exoplanets, the more we can understand how our own solar system came to be the way it is. With all seven planets Earth-sized, we can look at the different characterisitics that make each of them unique and determine critical connections between a planet’s conditions and origins.
For me as an astrobiologist and planetary scientist, these are indeed, interesting times! Until next time spacefans! Fly safe!
