EXOPLANETS

Starship Kepler—seeking out new worlds



By Amir Alexander
Science Writer, The Planetary Society


The 3 planets around the red dwarf star KOI-961 are all smaller than Earth and complete their orbits in less than 2 days.

The largest of these, orbiting the star Kepler 11, has at least 6 planets, while 6 other systems have at least 5 planets each.

According to Ragozzine, most of the planets in these systems are relatively small—only 1–3 Earth radii. The existence of so many such systems was a surprise, and accounts for the fact that many more planets were found than astronomers predicted before Kepler was launched.

But are they independent systems made up entirely of small planets, or is Kepler seeing the inner parts of planetary systems like our own—with small rocky planets orbiting close to the star and Jupiter-like giants further away? As yet, astro­nomers have no answer.

New Earths at last

Signs of intelligent life? Geometrically-arranged lights dot the surface of a moon orbiting a Jupiter-like planet somewhere in the depths of the galaxy.

For non-astronomers, the most exciting of Kepler's discoveries are undoubtedly the planets that are intriguingly like our own. In Dec 2011, members of the Kepler team introduced us to a planet orbiting in a Sun-like star's habitable zone—the ring-shaped region around a star where liquid water can exist and with it, potentially, life.

Designated Kepler 22b, the planet is more than twice Earth's mass, and astronomers aren't sure whether it is rocky like Earth and Mars or a ball of gas like Jupiter and Saturn. But the very discovery of a planet orbiting in conditions so similar to our own world suggested that the discovery of a real "Earth" might not not far behind.

Later that month the suggestion grew even stronger, when scientists at the Jet Propulsion Laboratory in California and the Harvard-Smithsonian Center for Astrophysics in Massachusetts announced that Kepler had discovered the first Earth-sized planets ever detected.

Finding such planets had been the chief goal of planet-hunting since the detection of the first gas giants in the 1990s—and now the long quest was finally at an end. The Sun-like star Kepler 20 is orbited by 5 planets, and 2 of them—Kepler 20e and Kepler 20f—are 87% and 103% the diameter of the Earth, respectively.

The planets orbit close to their star, and are very likely too hot for life as we know it. But what of life as we don't know it? If the crew of the Enterprise were ever to complete their 950-light-year journey to the Kepler 20 system, perhaps they could tell us more.

News of the discovery of Earth-sized planets around Kepler 20 was followed within a month by an announcement from NASA's Ames Research Center in California that Kepler had discovered 3 more hot rocky planets, the smallest no larger than Mars, orbiting a star designated KOI-961.

Now that planets the size of Earth and smaller have been detected, as well as a planet orbiting within the habitable zone of a Sun-like star, Kepler's next challenge is to combine the 2 by finding an Earth-sized planet that can sustain liquid water and, perhaps, life.

According to Ragozzine, such planets have probably already been detected. Among Kepler's trove of yet-to-be-confirmed planetary candidates are no fewer than 207 that are Earth-sized or smaller, and dozens of these are located in their star's habitable zone.

The Curiosity rover landed successfully on Mars on Aug 6, 2012. Unlike Kepler, which is searching for Earthlike planets light years away, Curiosity is looking for signs of potential life in our own solar system. Mars, at a minimum distance of only 55 million kilometers from Earth, is practically in our planetary back yard.

According to Ragozzine, it is almost certain that the majority of them will ultimately be confirmed as planets. When that happens, scientists will have found the Holy Grail of planet-hunting—Earth-sized worlds orbiting in the habitable zone of their stars—true alien "Earths."

Kepler effectively revolutionized our knowledge of our place in the universe, but it has now reached a crossroads. Within the next few months NASA will decide whether to end the mission by the end of 2012 or extend it for several more years.

Planet hunters around the world are anxious to see the mission continue, pointing out that only an extended mission will be able to find planets which, like Earth, take a year to orbit their stars.

A longer mission will also help determine whether the many planetary systems with small rocky planets also have much larger neighbors orbiting further out. This will tell us whether the architecture of our own solar system is commonplace in the galaxy, or whether we happen to reside in a strange outlier system.

With so much on the line, it is hard to accept that Kepler's life may be cut short. And yet, with many competing demands on a shrinking NASA budget, that is a realistic possibility. But, even if Kepler is not renewed, it will go down in history as a mission that opened humanity's eyes to the universe.

For the first time it showed us a galaxy full of planets and planetary systems, some very different and some tantalizingly like our own. It is a universe richer and more diverse than anyone could have imagined back in the 1960s, and yet one in which Capt Kirk and Mr Spock would feel quite at home—the Star Trek universe.

Amir Alexander is a science writer for The Planetary Society and teaches the history of science at UCLA.


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