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Planetary News: Extrasolar Planets (2006)Discovery of 16 Possible Planets Suggests Billions Still to ComeBy Amir Alexander5 October, 2006 In what could be a major breakthrough in the search for extrasolar planets, a team of astronomers announced today the discovery of 16 planet candidates in the central bulge of our galaxy. In an article in the October 5 issue of Nature magazine, lead author Kailash C. Sahu of the Space Telescope Science Institute and his colleagues describe how they used the Hubble Space Telescope to observe a batch of stars near the central bulge of the galaxy, at distance of up to 26,000 light years – ten times farther away than any planet previously found. The detections, they point out, not only add to our knowledge of specific planets, but also strongly suggests that planets are just as plentiful in the star-rich regions near the center of the galaxy as they are in our neighborhood on the galactic rim. The discovery was made by means of photometry, a method that yielded meager results in the first decade of extrasolar planet detection, but has been rapidly gaining ground in the past few years. All but 10 of the 200 odd extrasolar planets discovered so far were detected with the spectroscopic (or radial velocity) method, which measures the periodic shift in the spectrum of a star as it rocks to the tug of an orbiting planet. The success of this method is indisputable, but it tends to favor bright nearby stars, which can be observed for long stretches one after the other. In contrast, photometry observes thousands of stars simultaneously. With photometry, scientists look for a periodic dip in a star's luminosity that would indicate that an object is passing directly in front of the star as seen from Earth. Since the dip in the star's light curve is directly proportional to the diameter of the transiting object, the size of the dip provides a good estimate of the object's size. A sharp dip indicates presence of another star passing in front of the star as seen from Earth, but a slight dip indicates that the transiting object may well be a planet. "For the future, transits are the name of the game" in extrasolar planet research, said Alan Boss of the Carnegie Institution at a NASA press conference, called to discuss the latest discovery.
For their study, Sahu and his collaborators made use of the remarkable capabilities of the Hubble Space Telescope. For seven continuous days, between February 23 and 29, 2004, Hubble set its sights on a region of space known as the "Sagittarius I Window" of the Galactic bulge. The region contains no less than 180,000 stars bright enough for Hubble to observe, each of which was examined for evidence of a planetary transit. After carefully screening out candidates that seemed more likely to be transiting stars than planets, Sahu and his collaborators were left with 16 different objects that showed every sign of being true planets. Nevertheless, the authors were cautious. In itself, the transit (or photometric) method gives only the relative size of the transiting object as compared with the star. Since the size of the star can be known with considerable accuracy from its spectrum, it is quite easy to determine the absolute size of the orbiting object as well. The problem is that the diameters of some brown dwarfs and low mass stars are no greater than those of giant planets. The difference is that a stars' mass is many times greater than that of a planet, but of mass, the transit system provides no clue. How can we be certain then that the 16 supposed "planets" are not, in fact, stars? This is where the tried and true spectroscopic (or radial velocity) method comes to the rescue. Whereas photometry measures size, but not mass, spectroscopy measures mass, but not size. Working together they can provide an accurate estimate of both.
Knowing this to be the case, Sahu and his colleagues booked time on one of the most sensitive spectrographs on Earth: the UVES echelle at the European Southern Observatory's 8-meter telescope in Cerro Paranal, Chile. Although when working on its own spectroscopy's detection range is limited to several dozen light years, it can nevertheless yield excellent results at much greater distances if one knows exactly where to look. Between June 22 and 25, 2004, the authors pointed the giant telescope precisely at their 16 candidates, and measured for any signs of the telltale wobble. In 14 of the cases, the results were inconclusive, as the spectrograph was not able to separate a clear signal from the background noise, but for two stars significant results were obtained. In one, designated SWEEPS-04 (for Sagittarius Window Eclipsing Extrasolar Planet Search), a signal was detected, but no wobble. This is a significant result because it places an upper limit on the mass of the orbiting object. If it were greater than 3.8 Jupiter masses, a wobble would undoubtedly be registered. As it is, at under four Jupiter masses, SWEEPS-04 is unquestionably a planet. For the second object, designated SWEEPS-11, a wobble was indeed detected in the orbiting star, indicating a mass of around 9.7 Jupiters. While this places the object at the upper echelon of planetary masses, it is still smaller than the 13 Jupiter masses needed to initiate a nuclear reaction in the object's core. Therefore, by the accepted criteria, SWEEPS-11 is also a planet. As for the rest, Sahu and his fellow authors are convinced that many of them are planets as well. Using very conservative assumptions, they calculated that there is a very high probability that at least 7 of the 16 candidates are planets. Informally, Sahu said he believed that the number was actually considerably higher. One of the great surprises of the study, was the discovery of a new type of planet. Many of the planets discovered in the past have been "hot Jupiters," meaning that that they are gas giant planets that orbit very near to their star, completing each revolution in a matter of days. None however, was known to orbit in less than 1.2 days. Surprisingly however, a full 5 out of the 16 new candidate planets complete their orbit in less than a single day, and one of them, SWEEPS-10, completes it in a mere 10 hours. Appropriately, the authors dubbed the new class "ultra-short-period planets," or USPP's for short. Noting that USPP's occur only around relatively small and cool stars, the authors speculate that the heat of larger and hotter stars would have destroyed planets orbiting so close by.
"Discovering the very short-period planets was a big surprise" said Sahu. "Our discovery also gives evidence that planets are as abundant in other parts of the galaxy as they are in our own solar neighborhood" he added. This last insightmay indeed be the most significant result to come out of the study. By strongly suggesting that planets are more or less evenly scattered across the galaxy, it makes it possible to roughly estimate the number of planets in the Milky Way. By extrapolating from the more than 200 planets discovered so far, the authors conclude that there are a staggering 6 billion Jupiter-sized planets in our galaxy alone. "What this study has shown," said author Mario Livio of the Space Telescope Science Institute, "is that there are many, many planets out there, increasing the odds that we will find another Earth." Alan Boss agreed: "planets are everywhere" he said, "and orbiting all sorts of stars." "We are finally defining the terms of the Drake equation," he added, referring to the famous formula for estimating the number of communicating alien civilizations. "Sadly, Carl Sagan is not with us to see all the discoveries that have been made in the 10 years since his death." |
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