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Planetary News: Kepler (2011)Kepler Discoveries Suggest a Galaxy Rich in LifeBy Amir AlexanderFebruary 3, 2011
The search for distant Earths in the depths of space took a giant leap forward with the discovery of 5 Earth-size planet candidates orbiting in the habitable zone – the region around a star where liquid water is stable. If follow up observations confirm the discovery, the five planets would be among the first ever detected which could have water on their surface, and possibly – life. The detection of possibly life-friendly planets was one of several remarkable discoveries announced in a February 2 NASA press briefing on the latest results from the Kepler space telescope, based on observations conducted in its first 4 months of operation. Overall Kepler had found 1,235 new planetary candidates of all sizes, 68 of which are Earth-size, and 54 of which orbit in the habitable zone of their star. 170 of the candidates show evidence that they are part of multiple-planet systems, and one of these, designated Kepler 11, has no less than 6 transiting planets. This makes Kepler 11 the only system to date with more than 3 transiting planets, as well as the most planet-rich system known outside our solar system. To understand just how dramatically Kepler’s has transformed the search for exoplanets, consider that in the 15 years that followed the discovery of the first exoplanet in 1995 astronomers had detected around 500 planets. Kepler, in contrast, has already detected two and a half times that number, and that between May 12 and September 17 of 2009 alone! Add to that the fact that most of the planets found using ground-based methods have been giants, closer to the size of Jupiter than to Earth. Only very slowly in recent years have planet hunters managed to improve the sensitivity of their searches to the point where they are capable of detecting some planets of only a few Earth masses. Compare that to Kepler, which has already detected a planet smaller than the Earth, and several planets only slightly bigger. The quantum leap in the amount and quality of exoplanet data streaming from Kepler has left planet hunters breathless: “Kepler has blown the lid off everything we know about exoplanets” said veteran planet hunter Debra Fischer of Yale University.
Launched in March of 2009, Kepler trails the Earth in its annual journey around the Sun at a distance of around 18 million miles. Unlike space observatories like the Hubble and the Spitzer, which track astronomical phenomena in different parts of the sky, Kepler has its sensitive camera permanently fixed upon a single patch of sky and 156,000 stars within it. With its unblinking gaze it is searching for the slight but regular dimming of a star’s brightness, indicating that a planet has “transited” in front of it. Whenever such an event occurs in one of the stars under Kepler’s watch, the spacecraft’s sensitive instruments will almost certainly detect it. The slight dip in a star’s brightness that indicates a transit provides Kepler with a good estimate of the candidate planet’s diameter, but says nothing of its mass. Without this crucial figure scientists cannot be sure that the candidate they found is indeed a true planet. To find a candidate planet’s mass, astronomers most often turn to the tried and true radial velocity method, which makes use of large Earth-bound telescopes to detect the motion of a star as it rocks to the pull of an orbiting planet. Working together, the two methods can confirm that both the size and mass of the candidate is truly that of a planet. It also provides scientists with the average density of the planet, a crucial clue as to its composition. In some cases, however, planet hunters do not need to wait for the difficult radial velocity measurements to determine the mass of a planet detected by its transit. When two or more planets orbit a star in close proximity, they often interfere with each other, causing slight irregularities in the time it takes them to complete each orbit. From this scientists can calculate the ratio between the mass of the planets and that of their star, and since the stellar mass is known with considerable accuracy from existing models, the mass of the planets can be easily calculated. Such for example was the case with Kepler 9, a 3-planet system announced by the Kepler team in August of 2010. Such is also the case with the newly detected 6-planet system at Kepler 11, where scientists have confirmed that the planets’ masses range between around 3 and around 12 Earths, or roughly the mass of Uranus. In addition to the number of planets in the Kepler 11 system, the system is also remarkable for its compactness: the 5 inner planets orbit closer their star than Mercury – the innermost planet in our system – is to the Sun. The inner 5 also orbit in almost precisely the same plane, leading Kepler team member Jack Lissauer of NASA Ames to callthe system “flat as a compact disk.” The 6th planet is somewhat further removed, orbiting at a distance between that of Mercury and that of Venus, and on a slightly different plane. “We never thought we’d see this many sizable planets so close to each other” said Lissauer, adding that the discovery would provide important insights into the formation and migration of planets.
The discovery of 54 planets orbiting in the habitable zone of their stars has also stirred the planet hunting community by suggesting just how common planets bearing liquid water might be. “The search for planets is motivated by the search for life” said Fischer, and in that regard there is nothing more significant that the detection of possible water-bearing planets in deep space. Notably, only 5 of the 54 detected by Kepler appear to be in the size range for rocky planets, which could potentially have oceans and lakes on its surface. But that is no reason to give up on the others. These, said Kepler principal investigator William Borucki of NASA Ames, while not suitable for bearing water themselves, might have rocky moons where water can indeed flow on the surface. And where there is water, there just might be life. The 54 habitable-zone planets detected by Kepler all orbit stars that are substantially smaller than the Sun. As a result, the habitable zones are closer to the stars, and the planets’ orbital period consequently shorter. The detection of Earth-like planets orbiting in the habitable zone of Sun-like stars will take a few more years, simply because Kepler will need to observe at least 3 transits, which for a planet with an Earth-like orbit will take around 3 years. Even so, the implications of the spacecraft’s discoveries are already transforming our view of the universe and of ourselves within it. “Kepler can only see 1/400 of the sky” noted Borucki. “[It] can only find a small fraction of the planets around the stars it looks at,” because most orbiting planet do not transit, and are therefore invisible to the probe. “If you account for those two factors our results indicate that there must be millions of planets orbiting the stars that surround our suns.” And if planets are plentiful in the Milky Way, then life could be as well. “Kepler, said Borucki,” is the first step towards the detection of life in our galaxy.” |
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