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Planetary News: Extrasolar Planets (2004)Two Neptune-Size Extrasolar Planets Found
By Amir Alexander |
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A "Hot Jupiter" Credit: G. Bacon (STScI/AVL) |
The race to find distant new “Earths” heated up considerably yesterday with the detection of two of the smallest planets ever detected outside the Solar System. Whereas previous extrasolar planets discovered were almost exclusively gas giants, similar to Jupiter and Saturn and between 100 and 300 times the mass of the Earth, the newly found planets are much smaller, only 15 to 20 Earth masses. This places them in the mass range of Uranus and Neptune, somewhere between the gas giants and the small rocky planets of the inner Solar System. “We can’t see the Earth-like planets yet, but we can see their big brothers,” summed up Paul Butler of the Department of Terrestrial Magnetism at the Carnegie Institute.
One of the planets was found by the veteran planet-hunting team led by Butler and Geoff Marcy of U.C. Berkeley, the group responsible for detecting the majority of the 135 or so planets discovered to date beyond our Solar System. The planet orbits the M class red dwarf star Gliese 436, located only 33 light years away, in our own galactic neighborhood. Like many of the known extrasolar planets, Gliese 436’s companion remains very close to its star, completing each revolution in a mere two and a half days at a distance of 4.1 million miles. Most significantly, the planet’s minimum mass is only 21 Earth masses, equal to 1.2 “Neptunes.”
In addition to its small size, the planet is also significant because Gliese 436 is only the second red dwarf star known to have a planet. Red dwarfs are the most common stars, comprising around 70% of all the stars in the galaxy, and therefore they could be an important source of planetary discoveries. Even now, over 150 nearby red dwarfs are regularly tracked by Marcy’s team. Red dwarfs, however, are relatively small (Gliese 436 is two fifth the mass of the Sun) and extremely dim, at only 2% the brightness of the Sun. Because they are so faint they are very difficult to observe, and it is especially difficult to measure they light output with sufficient precision for planetary detections. It is also possible, said Butler - and the new discovery seems to support this view – that small stars like M-dwarfs tend to possess small planets, which are naturally harder to detect. Now that Neptune-mass planets are within the detection range of planet hunters, it is quite possible that many more planets will be discovered orbiting red dwarfs.
Like the vast majority of the distant planets found so far, Gliese 436’s companion was detected by the spectroscopic or “radial velocity” method. In this technique, scientists using extremely sensitive spectrometers measure the periodic changes to a star’s light spectrum as it wobbles back and forth to the tug of an orbiting planet. The more massive the planet is, the more noticeable and more easily detectable the wobble, which accounts for the fact that most of the planets detected so far have been Jupiter-size giants. It was the introduction of a new generation of spectrometers, that made possible the detection of these low mass planets.
Butler, Marcy, and their collaborators, which include Deborah Fischer of the U.C. Berkeley and Steven Vogt of U.C. Santa Cruz, found the planet by monitoring 950 stars in the solar neighborhood at the W.M. Keck Observatory in Mauna Kea, Hawaii. They managed to detect the small planet because Gliese 836 itself is a small star, and its wobble was therefore more pronounced than that of a larger star.
The second Neptune-mass planet was discovered by a team led by Barbara McArthur of the University of Texas orbiting the star 55 Cancri, about 41 light years away. Like the planet orbiting Gliese 836, 55 Cancri’s new planet is positioned very close to its star, and completes each orbit in just under three days. According to McArthur and her colleagues, the new planet’s mass is probably around 18 Earth masses, and could be as low as 14 “Earths.”
Remarkably, 55 Cancri was already known to be home to no less than three other planets. All three of the other planets around 55 Cancri are gas giants, with the outer one a true giant of around five Jupiter masses. But whereas two planets orbit very close to their star, one is more distant, completing each orbit in a Jupiter-like 4,520 days.
With the addition of the new Neptune-mass planet, 55 Cancri is now the first four-planet system ever discovered around a distant star. Like our own Solar System it exhibits a broad range of different orbits, from 3 days to 13 years, and now it also boasts a wide range of planetary masses, from the giant “super Jupiter” to the newly discovered “Neptune.” This is exactly what we would expect of a planetary system if we take our own Solar System as a model, explained Alan Boss of the Carnegie Institution, an expert on extrasolar planets who was not directly involved in the discoveries: numerous planets, diverse orbits, and a range of planetary masses. “This tells us that we should expect to find rather diverse contrary types in extrasolar systems, just as we find in our own Solar System” said Boss. “It encourages us to think that our analogue of our own Solar System to other planetary systems is a good analogue.”
McArthur and her colleagues detected the new planet by combining different planet-hunting approaches. They began the search by examining precise data from the Hubble Space Telescope, measuring the slight shifts in 55 Cancri’s position. This method, known as astrometry, is the oldest planet-hunting method, but no planets have yet been found based on it alone. Thanks to the effects of the long-orbit “super-Jupiter,” McArthur was able to detect measurable shifts in the location of 55 Cancri. From this she deduced the inclination of the planetary orbits as seen from Earth – a crucial factor for calculating the planets’ masses.
McArthur then turned to the radial velocity method, examining years of spectroscopic data collected by Butler and Marcy’s group as well as by the main European group of planet hunters, led by Michel Mayor of the Geneva Observatory. She supplemented this with more than 100 spectroscopic measurement of her own, using the University of Texas’s Hobby-Eberly telescope in West Texas. Combining all these strands of information, McArthur and her colleagues ran over 10,000 computerized simulations of 55 Cancri’s planetary system. The model that was clearly the best fit the masses of data included a Neptune sized planet orbiting the star in slightly less than three days at a distance of 5.6 million miles. Because the orbital plane’s inclination relative to the Earth was known from the Hubble’s astrometric measurements, McArthur could give a good estimate of the planet’s true mass: probably around 18 Earth masses, but possibly as low as 14.
Scientists do not know the composition of the new “Neptunes,” or whether they are more similar to gas giants or to small rocky planets like the Earth. Because of their size, which is similar to the most distant planets in our Solar System, Boss speculates that they may have formed far from their star and then migrated inwards. In that case they would be “Icy Giants,” much like Uranus and Neptune. Less probable, according to Boss, is the possibility that the planets are simply small gas giants, “gas midgets” perhaps.
The most intriguing possibility, said Boss, is that the planets did not migrate but were formed more or less where they were, close to their parent star. In that case they would be true rocky planets – the largest of a class of planets going down to Earth-size planets and smaller.
The discovery of the two Neptune-mass planets around Gliese 436 and 55 Cancri, following the announcement by a European team of the detection of a planet of similar mass around Mu Arae last week, represents an unexpected leap in the capabilities Earth-based planetary searches. Until recently most researchers thought that the discovery of terrestrial planets will have to wait for the space-based planet searches planned for the coming decade – Kepler, the Space Interferometry Mission, and the Terrestrial Planet Finder. No Longer: “it’s fair to say we are poised unexpectedly for the next step in planet discovery, namely finding truly Earth-Mass planets” said Marcy. We would like to search for “Super Earths” before any of the space missions are launched, added Butler.
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