Astronomie - Exoplanet-Entdeckung: 7 erdgroße Welten entdeckt bei nah gelegener Sonne



Exoplanets found orbiting Trappist-1 raise hope that the hunt for alien life beyond the solar system can start much sooner than previously thought


A huddle of seven worlds, all close in size to Earth, and perhaps warm enough for water and the life it can sustain, has been spotted around a small, faint star in the constellation of Aquarius.

The discovery, which has thrilled astronomers, has raised hopes that the hunt for alien life beyond the solar system could start much sooner than previously thought, with the next generation of telescopes that are due to switch on in the next decade.

It is the first time that so many Earth-sized planets have been found in orbit around the same star, an unexpected haul that suggests the Milky Way may be teeming with worlds that, in size and firmness underfoot at least, resemble our own rocky home.

The planets closely circle a dwarf star named Trappist-1, which at 39 light years away makes the system a prime candidate to search for signs of life. Only marginally larger than Jupiter, the star shines with a feeble light about 2,000 times fainter than our sun.

“The star is so small and cold that the seven planets are temperate, which means that they could have some liquid water and maybe life, by extension, on the surface,” said Michaël Gillon, an astrophysicist at the University of Liège in Belgium. Details of the work are reported in Nature.

While the planets have Earth-like dimensions, their sizes ranging from 25% smaller to 10% larger, they could not be more different in other features. Most striking is how compact the planet’s orbits are. Mercury, the innermost planet in the solar system, is six times farther from the sun than the outermost seventh planet is from Trappist-1.


Any life that gained a foothold and the capacity to look up would have a remarkable view from a Trappist-1 world. From the fifth planet, considered the most habitable, the salmon-pink star would loom 10 times larger than the sun in our sky. The other planets would soar overhead as their orbits required, appearing up to twice the size of the moon as seen from Earth. “It would be a beautiful show,” said Amaury Triaud at the Institute of Astronomy at Cambridge University

The researchers hope to know whether there is life on the planets “within a decade,” Amaury added. “I think we’ve made a crucial step in finding out if there’s life out there,” he said. “If life managed to thrive and releases gases in a similar way as on Earth, we will know.”

Astronomers reported last year what looked like three planets in orbit around Trappist-1, a star they named after the Trappist robotic telescope in the Chilean desert that first caught sight of the alien worlds. The telescope did not see the planets directly, but recorded the shadows they cast as they crossed the face of the star.

Any life that gained a foothold and the capacity to look up would have a remarkable view from a Trappist-1 world. From the fifth planet, considered the most habitable, the salmon-pink star would loom 10 times larger than the sun in our sky.
 Any life that gained a foothold and the capacity to look up would have a remarkable view from a Trappist-1 world. From the fifth planet, considered the most habitable, the salmon-pink star would loom 10 times larger than the sun in our sky. Illustration: Nasa/JPL-Caltech

The discovery prompted more sustained observations from the ground and space. Nasa’s Spitzer space telescope peered at the star for 21 days and, with data from other observatories, revealed a total of seven planets circling Trappist-1. The size of each planet was deduced from the amount of starlight it blocked out, while the mass was estimated from the way it was pushed and pulled around by other planets in the system. 

The planets are on such tight orbits that it takes between 1.5 and 20 days for them to whip around the star. At such proximity, most, if not all, will be “tidally locked”, meaning they show only one face to Trappist-1, just as one side of the moon always faces Earth. Some of the planets are thought to be the right temperature to host oceans of water, depending on the makeup of their atmospheres, but on others any hospitable regions may be confined to the bands that separate the light and dark sides of the planets.

Ignas Snellen, an astrophysicist at the Leiden Observatory in the Netherlands who was not involved in the study, said the findings show that Earth-like planets must be extremely common. “This is really something new,” he said. “When they started this search several years ago, I really thought it was a waste of time. I was very, very wrong.”

The top row shows an artist’s conception of the seven planets of Trappist-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars.
 The top row shows an artist’s conception of the seven planets of Trappist-1 with their orbital periods, distances from their star, radii and masses as compared to those of Earth. The bottom row shows data about Mercury, Venus, Earth and Mars. Illustration: NASA/JPL-Caltech

Astronomers are now focusing on whether the planets have atmospheres. If they do, they could reveal the first hints of life on the surfaces below. The Hubble telescope could detect methane and water in the alien air, but both can be produced without life. More complex and convincing molecular signatures might be spotted by Nasa’s James Webb Space Telescope, which is due to launch next year, and other instruments, such as the Giant Magellan Telescope, a ground-based observatory due to switch on in 2023. But there is only so much that can be done from afar. “We’ll never be 100% sure until we go there,” said Gillon.


The conditions on planets so close to dwarf stars, which are known to release fierce bursts of x-rays and ultraviolet light, might not be the most conducive for life. But when the sun goes out in a few billion years, Trappist-1 will still be an infant star. It burns hydrogen so slowly that it will last another 10 trillion years, Snellen writes in an accompanying Nature article. That is more than 700 times longer than the universe has existed, so there is plenty of time yet for life to evolve.

David Charbonneau, a professor of astronomy at Harvard University who was not involved in the latest study, said a growing number of astronomers were getting excited about what he called “the M-dwarf opportunity” – the study of planets around such faint dwarf stars. “It’s a fast track approach to looking for life beyond the solar system,” he said.

M-dwarfs outnumber sun-like stars 12 to 1 in the Milky Way. In previous work with Nasa’s Kepler planet-hunting telescope, Charbonneau and his colleague Courtney Dressing, found that one in four of M-dwarfs stars hosts a planet that is similar in size and temperature to Earth. With the Trappist-1 observations, astronomers now know that Earth-like planets circle nearby dwarf stars that can be studied with instruments already in the pipeline. “This means we might be in the business of looking for aliens in a decade, and not, as others have envisioned, on a much longer timescale,” he said. 

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These seven alien worlds could help explain how planets form

The Earth-sized astronomical bounty circles a dim star that flew under the radar of exoplanet researchers.



An artist's illustration shows two Earth-sized planets spinning across the face of an M dwarf star called TRAPPIST-1.

Seven alien, Earth-sized worlds bask in the cool, red light of their parent star. The planetary menagerie exists around a star overlooked by other exoplanet hunters, although it is just 12 parsecs (39 light years) from Earth.

Astronomers have found other seven-planet systems before, but this is the first to have so many Earth-sized worlds. All of them orbit at the right distance to possibly have liquid water somewhere on their surfaces.

“To have this system of seven is really incredible,” says Elisa Quintana, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “You can imagine how many nearby stars might harbour lots and lots of planets.”

Some of the planets were announced last year, but the authors debuted five newfound ones in a paper published on 22 February in Nature1. Because the system is so close to Earth, astronomers can study the planets’ atmospheres relatively easily. That could reveal an astonishing diversity of worlds, ranging in composition from rocky to icy.

“This system is going to be one of the best laboratories we have for understanding the evolution of small planets,” says Zachory Berta-Thompson, an astronomer at the University of Colorado Boulder.

It’s also vindication for astronomers who hunt for planets around the cool, dim stars known as M dwarfs. These are the most common type of star in the Milky Way, but many exoplanet searches have focused instead on bigger and brighter stars that more closely resemble the Sun. Even NASA’s Kepler space telescope, which found most of the more than 4,700 planetary candidates known so far, turned to M dwarfs only recently. “These small stars had been completely overlooked,” says Michaël Gillon, an astronomer at the University of Liège in Belgium.

Magnificent seven

Gillon leads the TRAPPIST collaboration, which hunts for planets using two 60-centimetre telescopes: one in Chile and one in Morocco. They look for the faint dimming of a star’s light that occurs when a planet moves across its face. The team initially reported three planets around the star, known as TRAPPIST-1, last May2.

The team had caught only two glimpses of one of those planets, so they followed up on the faint signals with other telescopes. That process included 20 consecutive days when NASA’s Spitzer Space Telescope stared at the star. The resulting data revealed that what the scientists thought was a single planet was actually four that orbit their star roughly every 4, 6, 9 and 12 days.

Those four joined the two innermost planets, which whirl around the star once every 1.5 days and 2.4 days. The team also caught a hint of a seventh, more distant planet.

Gillon says that the six inner planets probably formed farther away from their star and then migrated inward. Now, they are so close to each other that their gravitational fields interact, nudging one another in ways that enabled the team to estimate each planet's mass. They range from around 0.4 to 1.4 times the mass of the Earth.


An artist's illustration of what TRAPPIST-1's seven planets might look like.

Closing in

The arrangement of so many Earth-sized planets so close together will be a bonanza for researchers who are working to compare how worlds evolve. Venus and Earth started out in similar conditions, but ended up in two highly different states; uninhabitable Venus is now choked under a dense blanket of clouds. The TRAPPIST-1 system probably has a similar variety of worlds. “If one of these planets hosts life and the adjacent one doesn't, why not?” asks Sarah Ballard, an astronomer at the Massachusetts Institute of Technology (MIT) in Cambridge.

“This is a Rosetta stone with seven different languages — seven different planets that can provide us with completely different perspectives on planet formation,” adds team member Julien de Wit, a data scientist at MIT.

Although at least some fraction of each planet could harbor liquid water, it doesn't necessarily follow that they are habitable. TRAPPIST-1 emits about the same amount of X-ray and ultraviolet radiation as the Sun does, which could chew away at any protective atmospheres the planets might have3. And the worlds are likely locked into orbits where the same hemisphere always faces the star, rendering them permanently half-lit and half-dark. That would make it much more challenging for life to thrive.

Other researchers are already using the Hubble Space Telescope to hunt for atmospheres on the TRAPPIST-1 planets. Kepler is also observing the system and will gather data that can better pin down the planetary masses, says Courtney Dressing, an astronomer at the California Institute of Technology in Pasadena. And the TRAPPIST team is building four new 1-metre-diameter telescopes in Chile to continue the work.

“For all the worlds that we see in science fiction, these are even more extraordinary,” says Hannah Wakeford, an exoplanet scientist at Goddard.

Quelle: nature


Update: 26.02.2017


Sagan Institute director explains what life could be like near Trappist-1

Lisa Kaltenegger, one of the world’s leading experts on exoplanets and the potential for life beyond earth, and director of the Carl Sagan Institute at Cornell University, explains why NASA’s discovery is exciting and what life might look like on the seven Earth-like planets discovered near Trappist-1 – planets likely to have very high ultraviolet radiation flux on the surface.

Kaltenegger has two papers (“UV Surface Habitability of the TRAPPIST-1 System,” currently under review at Monthly Notices of the Royal Society, and “Biofluorescent Worlds: Biological fluorescence as a temporal biosignature for flare star worlds,” forthcoming in The Astrophysical Journal) that discuss life under a very high ultraviolet radiation flux environment.


Video: Lisa Kaltenegger discusses how her team is searching for alien life,

Kaltenegger says:

“Finding multiple planets in the Habitable Zone of their host star is a great discovery because it means there can be even more potentially habitable planets per star than we thought. And finding more rocky planets in the habitable zone per star definitely increases our odds of finding life.

“Trappist-1 now holds the record for the most rocky planets in the habitable zone – our solar system only has two – Earth and Mars. Life is a definite possibility on these worlds, but it might look different because there’s likely to be very high ultraviolet radiation flux on the surface of these planets.

“How good or bad would such a UV environment be for life? Our paper, currently under review at Monthly Notices of the Royal Society, discusses just this scenario for the Trappist-1 system, examining the consequences of different atmospheres for life in a UV environment.

“We find that if the star is active, as indicated by the X-ray flux, then planets need an ozone layer to shield their surface from the harsh UV that would sterilize the surface. If the planets around Trappist-1 do not have an ozone layer (like a young Earth), life would need to shelter underground or in an ocean to survive and/or develop strategies to shield itself from the UV, such as biofluorescence.

“Atmospheric biosignatures such as methane, indicating adaptations by life, could be detected by the James Webb Space Telescope, launching in 2018, or the European Extremely Large Telescope, coming online in 2022.”

Quelle: Cornell University


Update: 3.03.2017


TRAPPIST-1 planets

Probing Seven Worlds with NASA's James Webb Space Telescope


With the discovery of seven earth-sized planets around the TRAPPIST-1 star 40 light years away, astronomers are looking to the upcoming James Webb Space Telescope to help us find out if any of these planets could possibly support life.


“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.”


Launching in 2018, one of Webb’s main goals is to use spectroscopy, a method of analyzing light by separating it into distinct wavelengths which allows one to identify its chemical components (by their unique wavelength signatures) to determine the atmospheric components of alien worlds. Webb will especially seek chemical biomarkers, like ozone and methane, that can be created from biological processes. Ozone, which protects us from harmful ultraviolet radiation here on Earth, forms when oxygen produced by photosynthetic organisms (like trees and phytoplankton) synthesizes in light. Because ozone is largely dependent on the existence of organisms to form, Webb will look for it in alien atmospheres as a possible indicator of life. It will also be able to look for methane which will help determine a biological source of the oxygen that leads to ozone accumulation.


The discovery of the planets in the TRAPPIST-1 system means that Webb will be able to use its immense capabilities on a relatively nearby system. Researchers recently identified three promising planets in the TRAPPIST-1 system – e, f and g – which orbit in the habitable zone and would make good candidates for Webb to study. Depending upon their atmospheric composition, all three of these Earth-like exoplanets could have the appropriate conditions for supporting liquid water. Because the planets orbit a star that is small, the signal from those planets will be relatively large, and just strong enough for Webb to detect atmospheric features. Shawn Domagal-Goldman, an astrobiologist at NASA’s Goddard Space Flight Center said, “Two weeks ago, I would have told you that Webb can do this in theory, but in practice it would have required a nearly perfect target. Well, we were just handed three nearly perfect targets.”

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.”

NASA is exploring the solar system and beyond to better understand the universe and our place in it. We’re looking to answer age-old questions, like how did our universe begin and evolve; how did galaxies, stars, and planets come to be; and are we alone.

Quelle: NASA

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