This past Thursday, NASA announced the discovery of Kepler-186f, which the agency's press release called "the first Earth-size planet orbiting a star in the 'habitable zone'--the range of distance from a star where liquid water might pool on the surface of an orbiting planet." The press release emphasized the significance of the find by noting that "the discovery of Kepler-186f confirms that planets the size of Earth exist in the habitable zone of stars other than our sun."
University of Michigan astronomer and physicist Fred C. Adams was one of the team of scientists that analyzed the data from the Kepler space telescope. In a press release, Adams said, "One of the most interesting questions in science is whether life can arise on other planets or, alternatively, if life on this planet is unique. The discovery of planets with Earth-like properties is one important link in the chain required to answer this question. And the discovery of the planet Kepler-186f is an important step toward finding a planet that is like our Earth."
In the team's paper, "An Earth-Sized Planet in the Habitable Zone of a Cool Star," which was published in the journal Science on Friday, April 18, 2014, reported that Kepler-186f most likely has a diameter eleven percent larger than Earth's. This makes it smaller than previous rocky planets discovered by Kepler in the habitable zones of their stars, all of which have had diameters more than forty percent larger than Earth's and thus less suitable candidates for life as humans know it.
In the NASA press release, the paper's lead author Elisa V. Quintana elaborated on the significance of the finding, saying, "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. Finding a habitable zone planet comparable to Earth in size is a major step forward."
Quintana, now a research scientist at the SETI (Search for Extra Terrestrial Intelligence) Institute at NASA's Ames Research Center in California, also has connections to the University of Michigan. She was a graduate student at the University of Michigan, where Adams served as her thesis adviser.
She emphasized the importance at a news conference on Thursday, where she was quoted by The Daily Galaxy. “Kepler 186f is the first validated, Earth-size planet in the habitable zone of another star. It has the right size and is at the right distance to have properties similar to our home planet.”
Kepler-186f orbits a star about forty-seven percent the diameter of Earth's sun. It is classified as an M dwarf, which means the star is dimmer and redder than Earth's sun. Despite being closer to its sun with a year of 130 days, the planet receives only one-third of the energy from its star than the Earth. At high noon, the star's brightness is estimated to be the same as that of our sun an hour before sunset. This puts the planet near the outer edge of the star's habitable zone, although near its outer edge.
While the star is dimmer and cooler than Earth's sun, it is one of the most common types of stars, as M dwarfs make up 70 percent of the stars in the Milky Way galaxy. This makes Quintana optimistic about finding signs of life around them.
"M dwarfs are the most numerous stars," said Quintana in the NASA press release. "The first signs of other life in the galaxy may well come from planets orbiting an M dwarf."
Kepler-186f is part of the Kepler-186 system, about 500 light-years from Earth in the northern constellation Cygnus (The Swan). It is the the fifth and outermost world orbiting the red dwarf Kepler-186. According to NASA, the four companion planets, Kepler-186b, Kepler-186c, Kepler-186d, and Kepler-186e, whiz around their sun every four, seven, 13, and 22 days, respectively, making them too hot for life as humans know it. These four inner planets all measure less than 1.5 times the size of Earth.
In the University of Michigan press release, Adams said, "We found that this solar system does seem to be stable, it can be formed under reasonable conditions, and the planet is likely to be rocky, or Earth-like, and not gaseous."
While Quintana repeatedly pointed out the importance of the discovery, she says the system may be too dim for follow-up surveys to find the composition of its atmosphere, even with next-generation telescopes such as the Webb space telescope. Still, she was optimistic, as she was quoted as telling SETI in the University of Michigan press release.
"However, our research tells us that we should be able to find planets around bright stars that will be ideal targets to observe with James Webb."
Despite Kepler-186f being the right size and distance from its star, it may still not have life on its surface, as Thomas Barclay, research scientist at the Bay Area Environmental Research Institute at Ames, and co-author of the paper, cautioned in the NASA press release.
"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. Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth."