Astronomers have found at least seven Earth-sized planets orbiting the same star 40 light-years away, according to a study published Wednesday in the journal Nature. The findings were also announced at a news conference at NASA Headquarters in Washington.
This discovery outside of our solar system is rare because the planets have the winning combination of being similar in size to Earth and being all temperate, meaning they could have water on their surfaces and potentially support life.
"This is the first time that so many planets of this kind are found around the same star," said Michaël Gillon, lead study author and astronomer at the University of Liège in Belgium.
The seven exoplanets were all found in tight formation around an ultracool dwarf star called TRAPPIST-1. Estimates of their mass also indicate that they are rocky planets, rather than being gaseous like Jupiter. Three planets are in the habitable zone of the star, known as TRAPPIST-1e, f and g, and may even have oceans on the surface.
The researchers believe that TRAPPIST-1f in particular is the best candidate for supporting life. It's a bit cooler than Earth, but could be suitable with the right atmosphere and enough greenhouse gases.
If TRAPPIST-1 sounds familiar, that's because these researchers announced the discovery of three initial planets orbiting the same star in May. The new research increased that number to seven planets total.
"I think we've made a crucial step towards finding if there is life out there," said Amaury Triaud, one of the study authors and an astronomer at the University of Cambridge. "I don't think any time before we had the right planets to discover and find out if there was (life). Here, if life managed to thrive and releases gases similar to what we have on Earth, we will know."
Life may begin and evolve differently on other planets, so finding the gases that indicate life is key, the researchers added.
"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 NASA's Science Mission Directorate. "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."
And as we've learned from studying and discovering exoplanets before, where there is one, there are more, said Sara Seager, professor of planetary science and physics at Massachusetts Institute of Technology.
Seager and other researchers are encouraged by the discovery of this system because it improves our chances of finding another habitable planet, like Earth, in the future, by knowing where to look.
What we know
The planets are so close to each other and the star that there are seven of them within a space five times smaller than the distance from Mercury to our sun. This proximity allows the researchers to study the planets in depth as well, gaining insight about planetary systems other than our own.
Starting closest to the star and moving out, the planets have respective orbits from one and a half to nearly 13 Earth days. The orbit of the farthest planet is still unknown.
Standing on the surface of one of the planets, you would receive 200 times less light than you get from the sun, but you would still receive just as much energy to keep you warm since the star is so close. It would also afford some picturesque views, as the other planets would appear in the sky as big as the moon (or even twice as big).
On TRAPPIST-1f, the star would appear three times as big as the sun in our sky. And because of the red nature of the star, the light would be a salmon hue, the researchers speculate.
The researchers believe the planets formed together further from the star. Then, they moved into their current lineup. This is incredibly similar Jupiter and its Galilean moons.
Like the moon, the researchers believe the planets closest to the star are tidally locked. This means that the planets always face one way to the star. One side of the planet is perpetually night, while the other is always day.
Based on preliminary climate modeling, the researchers believe that the three planets closest to the star may be too warm to support liquid water, while the outermost planet, TRAPPIST-1h, is probably too distant and cold to support water on the surface. But further observation is needed to know for sure.
How the discovery was made
TRAPPIST-1 barely classifies as a star at half the temperature and a tenth the mass of the sun. It is red, dim and just a bit larger than Jupiter. But these tiny ultracool dwarf stars are common in our galaxy.
Weird and wonderful planets beyond our solar system
Kepler-10b orbits at a distance more than 20 times closer to its star than Mercury is to our own sun. Daytime temperatures exceed 1,300 degrees Celsius (2,500 degrees Fahrenheit), which is hotter than lava flows on Earth.
This Jupiter-like planet in the HD-188753 system, 149 light-years from Earth, has three suns. The main star is similar in mass to our own Sun. The system has been compared to Luke Skywalker's home planet Tatooine in "Star Wars."
Kepler-421b is a Uranus-sized transiting exoplanet with the longest known year, as it circles its star once every 704 days. The planet orbits an orange, K-type star that is cooler and dimmer than our Sun and is located about 1,000 light-years from Earth in the constellation Lyra.
Astronomers discovered two planets less than three times the size of Earth orbiting sun-like stars in a crowded stellar cluster approximately 3,000 light-years from Earth in the constellation Cygnus.
This artist's conception shows a hypothetical planet with two moons orbiting in the habitable zone of a red dwarf star. The majority of the sun's closest stellar neighbors are red dwarfs.
Kepler-186f was the first validated Earth-sized planet to be found orbiting a distant star in the habitable zone. This zone a range of distance from a star where liquid water might pool on the planet's surface.
Kepler-69c is a super-Earth-size planet similar to Venus. The planet is found in the habitable zone of a star like our sun, approximately 2,700 light years from Earth in the constellation Cygnus.
The Kepler-444 system formed when the Milky Way was just 2 billion years old. The tightly packed system is home to five planets that range in size, the smallest is comparable to the size of Mercury and the largest to Venus, orbiting their sun in less than 10 days.
This artistic concept image compares Earth, left, with Kepler-452b, which is about 60% larger. Both planets orbit a G2-type star of about the same temperature; however, the star hosting Kepler-452b is 6 billion years old -- 1.5 billion years older than our sun.
An artist's concept image of the surface of the exoplanet TRAPPIST-1f. Of the seven exoplanets discovered orbiting the ultracool dwarf star TRAPPIST-1, this one may be the most suitable for life. It is similar in size to Earth, is a little cooler than Earth's temperature and is in the habitable zone of the star, meaning liquid water (and even oceans) could be on the surface. The proximity of the star gives the sky a salmon hue, and the other planets are so close that they appear in the sky, much like our own moon.
Artist's conception of the binary system with three giant planets discovered, where one star hosts two planets and the other hosts the third. The system represents the smallest-separation binary in which both stars host planets that has ever been observed.
This artist's impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to our solar system.
This artist's impression shows a view of the surface of the planet Proxima b.
An artist's rendering shows Earth-sized exoplanets TRAPPIST-1b and 1c in a rare double transit event as they pass in front of their ultracool red dwarf star, which allowed Hubble to take a peek at at their atmospheres.
Out of a new discovery of 104 exoplanets, astronomers found four similar in size to Earth that are orbiting a dwarf star. Two of them have the potential to support life. The craft depicted in this illustration is the NASA Kepler Space Telescope, which has helped confirm the existence of thousands of exoplanets.
This artist's impression shows a view of the triple-star system HD 131399 from close to the giant planet orbiting in the system. Located about 320 light-years from Earth, the planet is about 16 million years old, making it also one of the youngest exoplanets discovered to date.
An artistic impression of the planet Kepler-1647b, which is nearly identical to Jupiter in both size and mass. The planet is expected to be roughly similar in appearance. But it is much warmer: Kepler-1647b is in the habitable zone.
HD-106906b is a gaseous planet 11 times more massive than Jupiter. The planet is believed to have formed in the center of its solar system, before being sent flying out to the edges of the region by a violent gravitational event.
Kepler-10b orbits at a distance more than 20 times closer to its star than Mercury is to our own sun. Daytime temperatures exceed 1,300 degrees Celsius (2,500 degrees Fahrenheit), which is hotter than lava flows on Earth.
This Jupiter-like planet in the HD-188753 system, 149 light-years from Earth, has three suns. The main star is similar in mass to our own Sun. The system has been compared to Luke Skywalker's home planet Tatooine in "Star Wars."
Kepler-421b is a Uranus-sized transiting exoplanet with the longest known year, as it circles its star once every 704 days. The planet orbits an orange, K-type star that is cooler and dimmer than our Sun and is located about 1,000 light-years from Earth in the constellation Lyra.
Astronomers discovered two planets less than three times the size of Earth orbiting sun-like stars in a crowded stellar cluster approximately 3,000 light-years from Earth in the constellation Cygnus.
This artist's conception shows a hypothetical planet with two moons orbiting in the habitable zone of a red dwarf star. The majority of the sun's closest stellar neighbors are red dwarfs.
Kepler-186f was the first validated Earth-sized planet to be found orbiting a distant star in the habitable zone. This zone a range of distance from a star where liquid water might pool on the planet's surface.
Kepler-69c is a super-Earth-size planet similar to Venus. The planet is found in the habitable zone of a star like our sun, approximately 2,700 light years from Earth in the constellation Cygnus.
The Kepler-444 system formed when the Milky Way was just 2 billion years old. The tightly packed system is home to five planets that range in size, the smallest is comparable to the size of Mercury and the largest to Venus, orbiting their sun in less than 10 days.
This artistic concept image compares Earth, left, with Kepler-452b, which is about 60% larger. Both planets orbit a G2-type star of about the same temperature; however, the star hosting Kepler-452b is 6 billion years old -- 1.5 billion years older than our sun.
An artist's concept image of the surface of the exoplanet TRAPPIST-1f. Of the seven exoplanets discovered orbiting the ultracool dwarf star TRAPPIST-1, this one may be the most suitable for life. It is similar in size to Earth, is a little cooler than Earth's temperature and is in the habitable zone of the star, meaning liquid water (and even oceans) could be on the surface. The proximity of the star gives the sky a salmon hue, and the other planets are so close that they appear in the sky, much like our own moon.
Artist's conception of the binary system with three giant planets discovered, where one star hosts two planets and the other hosts the third. The system represents the smallest-separation binary in which both stars host planets that has ever been observed.
This artist's impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to our solar system.
This artist's impression shows a view of the surface of the planet Proxima b.
An artist's rendering shows Earth-sized exoplanets TRAPPIST-1b and 1c in a rare double transit event as they pass in front of their ultracool red dwarf star, which allowed Hubble to take a peek at at their atmospheres.
Out of a new discovery of 104 exoplanets, astronomers found four similar in size to Earth that are orbiting a dwarf star. Two of them have the potential to support life. The craft depicted in this illustration is the NASA Kepler Space Telescope, which has helped confirm the existence of thousands of exoplanets.
This artist's impression shows a view of the triple-star system HD 131399 from close to the giant planet orbiting in the system. Located about 320 light-years from Earth, the planet is about 16 million years old, making it also one of the youngest exoplanets discovered to date.
An artistic impression of the planet Kepler-1647b, which is nearly identical to Jupiter in both size and mass. The planet is expected to be roughly similar in appearance. But it is much warmer: Kepler-1647b is in the habitable zone.
HD-106906b is a gaseous planet 11 times more massive than Jupiter. The planet is believed to have formed in the center of its solar system, before being sent flying out to the edges of the region by a violent gravitational event.
They were largely overlooked until Gillon decided to study the space around one of these dwarfs.
The researchers used a telescope called TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) to observe its starlight and changes in brightness. The team saw shadows, like little eclipses, periodically interrupting the steady pattern of starlight. This is called transiting. The shadows indicated planets, and further observation confirmed them.
In July, the team was able to determine that two of the closest planets to the stars had atmospheres that were more compact and comparable to those of Earth, Venus and Mars by observing starlight through the planets' atmosphere.
By using a global network ground-based telescopes like TRAPPIST and space-based telescopes like Spitzer, the researchers continued looking toward the TRAPPIST system and were able to determine the orbital periods, distances from their star, radius and and masses of the planets.
What's next
Over the next decade, the researchers want to define the atmosphere of each planet, as well as to determine whether they truly do have liquid water on the surface and search for signs of life.
Although 40 light-years away doesn't sound too far, it would take us millions of years to reach this star system. But from a research perspective, it's a close opportunity and the best target to search for life beyond our solar system.
"If we learn something now, it can determine if we looked in the right place," Gillon said.
In 2018, the James Webb Space Telescope will launch and be positioned 1 million miles from Earth with an unprecedented view of the universe. It can observe large exoplanets and detect starlight filtered through their atmosphere.
The researchers are also searching for similar star systems to conduct more atmospheric research. Four telescopes named SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) based in Chile will survey the southern sky for this purpose.
This star system will probably outlive us because this type of star evolves so slowly. When our sun dies, TRAPPIST-1 will still be a young star and will live for another trillion years, Gillon said. After we are gone, if there is another part of the universe for life to carry on, it may be in the TRAPPIST-1 system.
"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."
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