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Planetary News: Extrasolar Planets (2006)Astronomers Track the Birth of a PlanetBy Amir Alexander19 October 2006 Our galactic neighbor, Epsilon Eridani, has long been the target of both scientific and literary speculation. Back in 1960, the star was one of two targets of the very first SETI search – Frank Drake's Project Ozma. A few years later, when Star Trek hit television screens across North America, Epsilon Eridani was designated as the sun of Vulcan, Mr. Spock's home world. Then, in the year 2000, a true planet was detected orbiting Epsilon Eridani – among the first extrasolar planets discovered, and the closest one to Earth. And now, the decades of close observation of the star have yielded a truly remarkable result: the first direct evidence that planets are formed from clouds of gas and dust swirling around young stars. The "disk theory" of the formation of planets is not new. As far back as 1755 the German philosopher Immanuel Kant had speculated that planets evolved from the condensation of disks of matter rotating around stars. The theory, known at the time as the "nebular hypothesis," was given its first scientific formulation by the French polymath Pierre-Simon Laplace in 1796. Since that time, the hypothesis has been favored by some astronomers, but it was only in the second half of the 20th century that it became the generally accepted and quantifiable theory of planetary formation. The most persuasive evidence for the theory came from the only planetary system known at the time -- our own solar system, where the planets all orbit in the same direction and on more-or-less the same plane. The conclusion that the planets were once part of a single disk of matter revolving around the Sun seemed a natural one. Despite the near-universal acceptance of the disk theory of planetary formation, it was not until 1983 that the satellite IRAS (Infrared Astronomical Satellite) provided the first direct evidence in support of the theory, by detecting a cool cloud of gas and solid particles surrounding the star Alpha Lyrae. The following year, the 2.5-meter telescope at Las Campanas in Chile produced one of the most famous pictures of 20th century astronomy, by directly imaging just such a "protoplanetary disk" surrounding Beta Pictoris. In the 1990s, the Hubble Space telescope has imaged hundreds of similar disks, demonstrating that protoplanetary disks are a very common, if not universal feature of young stars. With the subsequent detections since 1995 of more than 200 planets orbiting distant stars, scientists began looking for planets that are orbiting inside an existing protoplanetary disk. This would for the first time provide direct evidence that planets are indeed born of these swirling clouds of gas and dust.
As it turned out, finding such a planet isn't easy. The problem is that although protoplanetary disks can be imaged directly by Hubble as well as by Earth-based telescopes, extrasolar planets cannot. Planets are only known indirectly through the effect they have on their star's spectrum, brightness, or position -- depending on the detection method used. The vast majority of the planets discovered to date orbiting other stars have been found by means of Doppler spectroscopy (also known as the radial velocity method), which measures the cyclical changes in the spectrum of a star as it rocks back and forth to the tug of an orbiting planet. A drawback of this method is that it only measures one axis of the star's movement – towards Earth or away from it. This means that it is always possible the plane of the star's motion is tilted to when viewed from the Earth, but we only detect that component of its movement that is along our line of site. Since the star's wobbling motion takes place on the same plane as the orbits of its planets, this means that the orbital plane of a distant planetary system can never be known by spectroscopy alone. For scientists looking to observe the birth of a planetary system, this is a problem. Even if they find a star that possesses both a planet and a disk, how are they to know that the two are orbiting together, on the same plane? As long as astronomers cannot determine the angle of the planet's orbital plane, they cannot compare it with the angle of the disk surrounding the star. A different method must therefore be found to determine the true tilt of a planet's orbital plane. In an article forthcoming in The Astrophysical Journal astronomers G. Fritz Benedict and Barbara E. McArthur of the University of Texas McDonald Observatory, George Gatewood of the University of Pittsburgh's Allegheny Observatory, and several collaborators, say they have found such a method: astrometry, the precise measurement of the location of a star in the sky over long periods of time. Using this method, they write, they have managed to establish that the orbital inclination of Epsilon Eridani's planet is the same as the inclination of the protoplanetary disk around the star. This almost certainly means that the planet is orbiting inside the disk of gas and dust that envelopes the star.
Epsilon Eridani is in some ways very similar to our Sun. Only 10.5 light years away it resides practically in our galactic backyard, and its mass is estimated at 83% of the Sun. The main difference between the two is their age: whereas the Sun has reached the mature middle age of 4.5 billion years, Epsilon Eridani is a sprightly 800 million years old. Because of its youth, Epsilon Eridani is still surrounded by a swirling disk of gas and dust, whereas our Sun had lost its own disk billions of years ago. Nevertheless, according to prevailing models, Epsilon Eridani is old enough for its disk to have given birth to actual planetary companions, and it therefore came as no surprise that just such a planet was discovered orbiting the star in 2000. The planet, as spectroscopic measurements showed, is a Jupiter-like gas giant that orbits its star approximately once every seven years. Such a long period meant that Benedict and his colleagues would have to look at many years of data to establish the regular movement of the star, and consequently the orbital tilt of the planet. The core of their data consisted of three years of exceedingly accurate astrometric measurements of the position of Epsilon Eridani by the Hubble Space telescope, from 2001 to 2003. While this may sound like a lot, Benedict noted, having only three years of data for a seven-year orbit is "a bit worrisome." So the authors looked at more data, which although not quite as accurate as the Hubble measurements, took place over a longer period of time: 14 years of astrometric measurements of Epsilon Eridani by the Multichannel Astrometric Photometer at the University of Pittsburgh's Allegheny Observatory, dating back to 1988. To this they added spectroscopic measurements going back a full 25 years, which firmly established the periodic movement of the star. With such a long base of both astrometric and spectroscopic observations, Benedict and his colleagues could now track the true periodic motions of Epsilon Eridani as its planet traveled through its orbit. They first concluded that the planet was real, and that its actual orbital period of the planet is 6.85 years. This is not quite as self evident as it sounds, explained Benedict, because sometimes surface phenomena on a star can mimic the spectrographic effects of an orbiting planet. This is particularly true in the case of a young and volatile star like Epsilon Eridani, and the danger is always acute when dealing with a long-period planet, where the data covers only part of a single orbit. As a result, although most astronomers accepted the reality of Epsilon Eridani's planet, some nevertheless had their doubts.
The authors then observed that the orbital inclination of the planet, as seen from Earth is quite substantial – a full 30 degrees. This result is important for calculating the planet's mass, because it reveals what component of its real motion is seen from Earth as the "back and forth" wobble of the star detectable in spectroscopic measurement. In this case, the mass of the planet turned out to be 1.55 Jupiters, well within the range of planetary masses. But the 30 degree tilt is important for another reason: it very closely matches the known tilt of the protoplanetary disk surrounding Epsilon Eridani. And this means that the giant Jupiter-like planet known as Epsion Eridani_b is very likely the first planet ever detected orbiting within the cloud of gas and dust from which it formed. Our world was a very different place when Kant and Laplace speculated on the origins and birth of planets, and since their time our knowledge of planetary systems and of the universe as a whole has been transformed and turned on its head several times over. Remarkably, through it all, the theory of the formation of planets proposed by these two Enlightenment scholars has endured and thrived, fundamentally unchanged, growing ever more persuasive and yet waiting for a final proof. It took more than two centuries, but now its time has finally come: With Epsilon Eridani_b we are, at long last, observing a young planet moving through the cloud of gas and dust of which it was born. |
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