April 17, 2014
Astronomers Using Kepler Telescope Discover First Earth-Sized Exoplanet Within The Habitable Zone
[ Watch the Video: Kepler Discovers First Earth-Sized Exoplanet In Habitable Zone ]
Lawrence LeBlond for redOrbit.com - Your Universe Online
The discovery of this planet, called Kepler-186f, is confirmation that exoplanets the size of Earth do exist in the habitable zone of stars other than our sun. The habitable zone – often referred to as the Goldilocks Zone – is the range of distance from a star where liquid water can pool on the surface of an orbiting planet.
This is not the first planet that has been discovered within the habitable zone, but it is the first of near equal size to Earth. Other larger planets that have been found have been at least 40 percent larger than Earth and have been difficult for researchers to determine their exact composition.
"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," said Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington. "Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds."
Stephen Kane, of San Francisco State University, led an international team in the discovery of this rocky exoplanet that could potentially have liquid water on its surface. Since water is critical to life on Earth, astronomers believe the search for extraterrestrial life should focus on planets that orbit stars within the habitable zone and have the greatest chances for water to exist.
"Some people call these habitable planets, which of course we have no idea if they are," said Kane, an assistant professor of physics and astronomy. "We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets."
Kepler-186f is the fifth and outermost planet discovered orbiting around the dwarf star Kepler-186. All of the planets in this system were discovered using the transit system, which detects potential planets as they race around their host star, causing faint disturbances in the star’s light that Kepler can pick up.
Once Kepler-186f was confirmed as a planet, the team calculated the planet’s size based on the transit information available. Their calculations showed that Kepler-186f is slightly larger than Earth, measuring about 1.1 Earth radii. While they have got the measurement down, they said they do not yet know the mass of the planet, but are able to estimate it based on other planets with similar radii. As well, the composition is not yet known, but previous studies of similarly-sized planets labels means this one is likely to be rocky.
"We know of just one planet where life exists -- Earth. When we search for life outside our solar system we focus on finding planets with characteristics that mimic that of Earth," said Elisa Quintana, research scientist at the SETI Institute at NASA's Ames Research Center in Moffett Field, Calif., and lead author of the paper published today in the journal Science. "Finding a habitable zone planet comparable to Earth in size is a major step forward."
The star system where we find Kepler-186f is within the constellation Cygnus. The system, home to five planets in all, orbit a star that is about half the size and mass of our own sun. The star is classified as an M or red dwarf, a class of stars that makes up 70 percent of the stars in the Milky Way galaxy.
"M dwarfs are the most numerous stars," said Quintana. "The first signs of other life in the galaxy may well come from planets orbiting an M dwarf."
Kepler-186f orbits its star once every 130 days and receives one-third the energy from its star that Earth gets from the sun, which puts the planet at the outer edge of the habitable zone. On the surface of the planet, the brightness of its star at high noon is only as bright as our sun appears to us about one hour before sunset.
"Being in the habitable zone does not mean we know this planet is habitable. The temperature on the planet is strongly dependent on what kind of atmosphere the planet has," said Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at Ames, and co-author of the paper. "Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth."
The other four planets in the system all revolve around the host star from between four and 22 days, making all of them far too hot to support life as we know it. As well, the four inner planets all measure less than 1.5 times the size of Earth.
Knowing the mass and radii of a planet allows astronomers to calculate other features such as the planet’s density. Once the average density is known, then the composition can be determined, according to Kane.
"What we've learned, just over the past few years, is that there is a definite transition which occurs around about 1.5 Earth radii," he explained. "What happens there is that for radii between 1.5 and 2 Earth radii, the planet becomes massive enough that it starts to accumulate a very thick hydrogen and helium atmosphere, so it starts to resemble the gas giants of our solar system rather than anything else that we see as terrestrial."
Also, the size of planet influences its gravitational pull and the ability to pull in abundant gases like hydrogen and helium. At Kepler-186f’s size, there is little chance that it could have gathered up a thick layer of hydrogen and helium, meaning there is an excellent chance that it does have a rocky surface, Kane continued.
"We're always trying to look for Earth analogs, and that is an Earth-like planet in the habitable zone around a star very much the same as our Sun," said Kane, who is the chair of Kepler's Habitable Zone Working Group. "This situation is a little bit different, because the star is quite different from our sun."
"The diversity of these exoplanets is one of the most exciting things about the field," Kane said. "We're trying to understand how common our solar system is, and the more diversity we see, the more it helps us to understand what the answer to that question really is."
Steve Howell, Kepler’s Project Scientist at NASA and a coauthor of the new paper, maintained that there is currently no telescope big enough to be able to directly spot an exoplanet of this size and proximity to its host star. "However, what we can do is eliminate essentially all other possibilities so that the validity of these planets is really the only viable option."
To confirm that the Kepler Space Telescope picked out a candidate planet, the team used a technique that eliminated the possibility that either a background star or stellar companion could have been mimicking what Kepler detected.
The team obtained extremely high spatial resolution observations from the eight-meter Gemini North telescope on Mauna Kea in Hawaii using speckle imaging, as well as adaptive optics observations from the ten-meter Keck II telescope, Gemini’s neighbor on Mauna Kea. Together, the data from the telescopes allowed the team to rule out other sources and concluded that what Kepler detected was a signal from a small planet transiting its host star.
"The Keck and Gemini data are two key pieces of this puzzle," said Quintana. "Without these complementary observations we wouldn't have been able to confirm this Earth-sized planet."
"These Earth-sized planets are extremely hard to detect and confirm, and now that we've found one, we want to search for more. Gemini and Keck will no doubt play a large role in these endeavors."
"The observations from Keck and Gemini, combined with other data and numerical calculations, allowed us to be 99.98 percent confident that Kepler-186f is real," added Barclay. "Kepler started this story, and Gemini and Keck helped close it."