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Planetary News: Extrasolar Planets (2010)From the Ground and from Space, New Planetary Systems UnveiledBy Amir AlexanderAugust 27, 2010
Two nearly simultaneous announcements by scientists that they have detected entire planetary system deep in space have set the astronomical community abuzz. One discovery was made with data from the planet-hunting spacecraft Kepler, the other with a ground-based telescope in La Silla, Chile. In both cases, there are strong indications that the systems contain at least one small rocky planet, only slightly more massive than the earth. "Once again, exploring stars from the ground and from space has led to the discovery of distant worlds” said Bill Nye, Executive Director Designate of The Planetary Society. I cannot help but wonder; does anyone live there? Do they have instruments that can detect Earth? Planetary exploration changes us and the way we see our place in space." Scientists analyzing data from the spacecraft Kepler announced that for the first time Kepler, which is designed to search for planets that pass directly in front of their star, found more than one such “transiting” planet orbiting the same star. It is the first planetary system discovered by the transit method, and the marvelous sensitivity of the technique is once again on display. Along with two giant planets detected by Kepler orbiting the star there is a third one -- not yet confirmed but strongly suspected. This planetary candidate appears to be only 1.5 Earth masses, making it the lowest mass planet yet detected. 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 most space telescopes, which track different 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. During its three and a half year mission, and scientists believe that the spacecraft could discover hundreds if not thousands of new planets.
Kepler found the new planetary system around a Sun-like star now designated Kepler 9, about 2000 light-years from Earth. The light measurements collected by the spacecraft clearly indicated the presence of two large transiting planets: One, Kepler 9b, orbited the star every 19.2 days and the other, Kepler 9c, completed each orbit in just under 39 days. The fact that the ratio of the 2 planets’ periods is almost exactly 1:2 is significant here: it tells astronomers that the two planets are “in resonance,” meaning that through their gravitational pull they “shepherd” each other along their orbital path. When planets interact gravitationally in this way they also interfere with each other’s motion, resulting in slightly irregular periods. This means that each orbit might take a planet slightly longer or shorter time than average. Since the orbital period of a planet is the time that elapses between each of its transits Kepler should be able to detect this, and sure enough it did: Kepler 9b’s orbits varied by an average of 4 minutes, and Kepler 9c’s by an average of 39 minutes. “This discovery is the first clear detection of transit timing variations” in exoplanets, said Matthew Holman of the Harvard-Smithsonian Center for Astrophysics (CfA) who co-authored the Science Express article announcing the discovery. Measuring transit timing variations provides astronomers with much more than evidence of gravitational interplay between planets: it allows them to accurately estimate the mass of a planet. In most cases transit measurements provide an accurate measurement of a planet’s period and distance from its star. Crucially, from the degree of a star’s dimming during transit scientists can also accurately deduce the size of the planet’s disk. But transit measurements do not usually provide scientists with the all-important figure of the planet’s mass: for that they have to turn to the tried and true radial velocity technique, which is conducted using one of the world’s great ground telescopes.
But if, as is the case with Kepler 9, the transiting planets gravitationally interact with each other causing their orbital periods to vary, then it is possible to estimate their mass directly from the transit measurements. “The timing of the transits can be used to measure the mass ratio between the planets and the star” explained article co-author Darin Ragozzine of CfA. Since the mass of the star is known from stellar models, the mass of the planets can be deduced as well. Measured in this way both planets orbiting Kepler 9 proved to be Saturn-mass planets, the inner planet (Kepler 9b) coming in at 0.25 Jupiter masses, and the outer planet (Kepler 9c) measuring 0.17 Jupiter masses. Six radial velocity measurements taken with the giant Keck telescope in Hawaii later confirmed these figures. Transiting planets like Kepler 9b and Kepler 9c that show timing variations offer scientists a treasure trove of information about their system. The planet’s size and mass allow astronomers to calculate the planet’s density, which is a crucial clue for its composition. A low density planet is almost certainly a gas giant, whereas higher densities suggest the presence of ices and rocks. According to Alycia Weinberger of the Carnegie Institution, this information, along with the planets’ current orbits, can help scientists trace the evolution of the planetary system from its earliest beginning to its present state. Furthermore, orbital timing variations can point to the presence of additional planets in a system, even if these planets do not themselves transit. “Rich systems like this one will be the best laboratories for understanding how planets form, and how planetary systems evolve” said article co-author Tim Brown of the Las Cumbres Observatory Global Network.
If all this was not enough, the Science Express article offers one more tantalizing clue: hidden deep in the data for Kepler 9 is evidence of one more transiting planet, orbiting very close to its star and completing each revolution in a mere 1.6 days. The truly remarkable thing about this planetary candidate is its mass – only 1.5 times that of the Earth, making it the lowest mass exoplanet ever detected. If confirmed, this planet will bring Kepler ever closer to its goal of finding Earth-like planets deep in the vastness of space. While the Kepler team was unveiling a distant planetary system through transit measurements, a European group using radial velocity measurements announced the discovery of possibly the largest known planetary system orbiting a distant star. Using the HARPS spectrometer at the 3.6 meter telescope in La Silla, Chile, the team detected no less than 5 planets orbiting the star HD 10180, at a distance of 127 light-years from Earth. This in itself equals the number of planets orbiting 55 Cancri, the most planet rich system known to day, but the scientists believe there is more: “We also have good reasons to believe that two other planets are present” said team leader Christoph Lovis of the Geneva Observatory. If these are confirmed, the 7-planet system around HD 10181 will be nearly as large and complex as our own 8-planet solar system. “This remarkable discovery” said Lovis, “highlights the fact that we are now entering a new era in exoplanet research: the study of complex planetary systems and not just of individual planets.”
The 5 confirmed planets have a mass roughly that of Neptune, between 13 and 25 Earth masses. Their orbits periods range from 6 to 600 days, and their distance from their star ranges from a tiny fraction of the Earth-Sun distance to roughly one and a half times that distance. As in our solar system, the planets’ orbits are roughly circular, but in other respects the new system is quite different: It has many more large planets closer in to the star, and it appears to lack a true giant like Jupiter. But much like the Kepler discovery, it is the unconfirmed detections that might be most intriguing. One of these, explained Lovis, is a suspected Saturn-mass planet with an orbital period of 2200 days. “The other would be the least massive exoplanet ever discovered, with a mass of about 1.4 times that of Earth. It is very close to its host star, at just 2 percent of the earth-Sun distance. One “year” on this planet would last only 1.18 Earth-days.” With the aid of spacecraft orbiting the Sun, and large telescopes in the heights of the Andes, scientists are unveiling entire planetary systems hundreds and thousands of light-years away. Along with giant “Saturns” and massive “Neptunes,” these systems are also made up, it seems, of small rocky planets, hardly larger than our own. Can the detection of a true “Earth” in the depths of space be far behind? |
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