From Giodarno Bruno's heretical vision of Infinite Worlds in 1593 until 1992, our notion of star-orbiting planets beyond Earth’s solar system only existed in science fiction. That’s when astronomers Alex Wolszczan and Dale Frail used the 1,000-foot Arecibo radiotelescope to detect three planets orbiting a rapidly spinning neutron star -- the very first discovery of planets outside our solar system. This discovery opened the door to the current intense era of planet hunting by suggesting that planet formation could be quite common throughout the universe and that planets can form around different types of stellar objects.
in 2014, astronomers have discovered 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 discovery of Kepler-186f confirms that planets the size of Earth exist in the habitable zone of stars other than our sun.
Flash back to October of 2011, three planets -- each orbiting its own giant, dying Red Dwarf star -- were discovered by an international research team The new research shed light on the evolution of planetary systems around dying stars, and helped astronomers to understand how metal content influences the behavior of dying stars.
The three newly-discovered planetary systems are more evolved than our own solar system. "Each of the three stars is swelling and has already become a red giant -- a dying star that soon will gobble up any planet that happens to be orbiting too close to it," according to Wolszczan, professor of astronomy and astrophysics at Penn State.
"While we certainly can expect a similar fate for our own Sun, which eventually will become a red giant and possibly will consume our Earth, we won't have to worry about it happening for another five-billion years." Wolszczan also said that one of the massive, dying stars -- BD +48 738 -- is accompanied not only by an enormous, Jupiter-like planet, but also by a second, mystery object.
This past October, NASA’s Kepler observatory has spotted 20 planets that orbit cool, small stars — the largest such haul so far. These long-lived stars, known as K and M dwarfs, are ubiquitous in the Milky Way and could turn out to host numerous habitable planets.
California's SETI Institute believes that planetary systems orbiting red dwarfs — dim, long-lived stars that are on average billions of years older than our sun — are worth investigating for signs of advanced extraterrestrial life. The star that’s closest to our sun, Proxima Centauri, is a red dwarf. A variety of observing efforts, including Cornell's Pale Red Dot initiative, are looking for habitable planets around Proxima Centauri (shown above, including the recently discovered, Promxima b some 4 light years from Earth.
“This may be one instance in which older is better,” said astronomer Seth Shostak of California-based SETI, a private, non-profit organization which stands for Search for Extraterrestrial Intelligence. “Older solar systems have had more time to produce intelligent species.”
After the Kepler spacecraft experienced a mechanical failure in 2013 that made it impossible for it to keep observing its original targets, astronomers gave it a new mission, called K2. It now uses pressure from sunlight to help stabilize the craft. The latest observations with K2 revealed 87 planet candidates, on top of 667 previously announced candidates, almost all with sizes between those of Mars and Neptune.
Although the original Kepler mission examined many Sun-like stars, the majority of stars in our Galaxy are smaller, fainter, cooler stars, known as red dwarfs. Such stars make up nearly half the targets of the K2 mission.
“There are more than 250 of them within 30 light-years — all over the place — which is why some other astronomers here might call them the vermin of the sky,” says Courtney Dressing, an astrophysicist at the California Institute of Technology.
“Since these stars are the most common ones in the Galaxy, they help us learn how common life might be,” says Victoria Meadows, an astronomer at the University of Washington in Seattle.
Of the confirmed planets, 63 are smaller than Neptune, and a few could be even smaller than Earth. But these small candidates remain to be confirmed. Dressing believes that these are probably “false positives” caused by other phenomena such as cosmic rays or an instrumental glitch.
Five of the confirmed planet candidates are in or near their star’s ‘habitable zone’, the region that’s neither too close to the star, nor too far from it, for life to arise. In our Solar System, the zone is roughly between the orbits of Venus and Mars.
Red dwarf stars give off less energy than larger, hotter stars, so their planets’ habitable zones are closer in, often closer to their star than Mercury is to the Sun. Such planets transit frequently, some orbiting their star within just a few weeks, making it easier to use Kepler’s instruments to detect the tell-tale dimming of stellar light.
The focus on red dwarfs stems partly from the K2 mission’s constraints, which allow the astronomers less then three months to observe stars in its field of view before having to rotate the craft. Moving from field to field poses a challenge, but it also gives the team an opportunity to investigate more objects. “It’s fun to study a new set of stars every 80 days,” Dressing says.
Dressing’s research also paves the way for more sensitive future missions designed to look for Earth-sized planets, said Christa van Laerhoven, a planetary scientist at the Canadian Institute for Theoretical Astrophysics in Toronto. Such missions include NASA’s Transiting Exoplanet Survey Satellite, scheduled to launch in December next year.
Image top of page: Life in the universe might be even rarer than we thought. Recently, astronomers looking for potentially habitable worlds have targeted red dwarf stars because they are the most common type of star, comprising 80 percent of the stars in the universe. But a new study shows that harsh space weather might strip the atmosphere of any rocky planet orbiting in a red dwarf's habitable zone.
"A red-dwarf planet faces an extreme space environment, in addition to other stresses like tidal locking," says Ofer Cohen of the Harvard-Smithsonian Center for Astrophysics (CfA).