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Planetary News: Extrasolar Planets (2008)Rocky Planets Plentiful around Distant Suns, Study SuggestsBy Amir AlexanderMarch 10, 2008
Where should we look for rocky planets like our Earth orbiting alien stars? Practically anywhere, suggests a new study by University of Arizona astronomer Michael Meyer and an international team of collaborators, published last month in The Astrophysical Journal. According to the article at least 20%, and possibly as many as 60% of Sun-like stars are orbited by rocky planets. If this estimate is confirmed, terrestrial planets like our own might be far more common in our galaxy than scientists had suspected. This is welcome news for astronomers searching for extrasolar planets in the vastness of space. Finding a distant "Earth" is the holy grail of the search for extrasolar planets, but current methods are simply not sensitive enough to find such small objects orbiting in the vicinity brilliant stars. As a result, although planet hunters have detected more than 200 distant planets so far, the vast majority of these are gas giants, dozens or hundreds of times more massive than Earth, and not one has been smaller than 5 Earth masses. The detection of Earth-mass planets will likely have to wait several years for the launch of space-based planet-searches, and Meyer's article provides an encouraging estimate of what these missions could uncover. While it is not as yet possible to detect actual rocky planets orbiting Sun-like stars, Meyer and his colleagues noted that it just might be possible to detect something almost as significant: the telltale signs of rocky planets in the process of formation. This is because all planets are formed from the protoplanetary disk of gas and dust that typically envelopes young stars, but what type of planet is formed depends largely on its distance from the star. According to current models, planets around Sun-like stars forming at distances beyond 5 Astronomical Units (AU) from their star – each AU representing the average distance between Earth and the Sun – are almost certainly gas giants. Terrestrial planets are formed from warmer particles swirling closer to their star. Since the detection of the first protoplanetary disks in the 1980's astronomers have become very good at finding them even when they are too distant to be observed directly. They rely on the fact that the disk absorbs heat from its star, and then expels it in the form of infrared radiation. This produces an "infrared excess" in the star's spectrum, which to astronomers is a sure sign that the star is accompanied by a swirling disk of debris.
But if an infrared excess in itself points to the presence of a protoplanetary disk, then precise measurements of this excess might yield some valuable information about the disk's composition. In particular, the different wavelengths that compose the infrared radiation may provide some important clues as to how far from the star the debris cloud is swirling. The closer the dust is to the star the hotter it is, which means shorter-wave radiation. Debris swirling at terrestrial distances from its star would be "warm" by galactic standards – between minus 280 and 80 degrees Fahrenheit, which is detectable as radiation at wavelengths around 24 microns. If significant levels of such radiation can be measured within the excess infrared spectrum of a disk, the authors reasoned, it would be a very strong indication that terrestrial planets are indeed forming in these regions. To determine the precise wavelengths of excess infrared radiation from stars Meyer and his colleagues turned to the Spitzer Space Telescope, designed for observations in the infrared part of the spectrum. They focused on a sample of 309 Sun-like stars and divided them into six groups based on their age – from the very young 3 to 10 million year-olds to the mature 1 to 3 billion year-olds. Our stately middle-aged Sun, by comparison, came into being 4.6 billion years old. For each star in the sample they compared the strength (or "flux") of the 24 micron warm-dust radiation to the sizzling 8 micron radiation that indicates the presence of particles heated to over 2000 degrees Fahrenheit. When the measurements were in, they had a story to tell. 30 stars out of the sample of 309 showed a statistically significant excess of 24 micron radiation, ranging from 13% to over 100% of the 8 micron radiation. But what interested the researchers most was the age of the "warm dust" planets: "We found that about 10 to 20 percent of the stars in each of the four youngest age groups show 24 micron emissions due to dust" Meyer said. "But we don't often see warm dust around stars older than 300 million years. The frequency just drops off."
The significance of this result is that it parallels current theories of the formation of rocky planets. The time periods at which warm dust is present and then disappears, explained Meyer, is "comparable to the time scales thought to span the formation and dynamical evolution of our own solar system." During our solar system's infancy collisions between planet-forming rocks and planetisimals produced vast amounts of warm dust in the protoplanetary disk at distances of 1 to 5 AU from the Sun. In time, some of the dust particles accreted to form the rocky planets and the rest were cleared away by the planets' gravitational effects. When the process was complete and full-grown rocky planets were orbiting the Sun, the cloud of warm gas had dissipated forever. Meyer and his colleagues' observations with the Spitzer suggest that the same process is taking place around many Sun like stars. When the stars are young, an excess of 24 micron emissions indicates the presence of massive amounts of warm dust coalescing into rocky planets. But by the time the stars reach an age of about 300 million years the process is complete, the rocky planets are formed and the warm dust dissipated. That was the case with the Sun over 4 billion years ago, and that also seems to be the case today with many Sun-like stars. But what portion of Sun-like stars actually forms rocky planets? The answer, said Meyer, depends on how one interprets the results of their observations. It is possible, he reasons, that the stars in one age group who show the presence of warm dust, are essentially the same "warm dust" stars that appear in the slightly older and slightly younger age groups. That would mean that a 3 million year-old star showing the presence of such particles would still show them at 10 million and 30 million years. Such a conservative reading of the data suggests that around 1 in 5, or 20%, of Sun-like stars are likely forming rocky planets. A more optimistic reading, however, assumes that the stars surrounded by warm dust in each age group are different stars from the ones that show up in the older and younger groups. This could be so if the most massive disks form their planets quickly and early, completing the process before thinner disks begin their own planet-forming work. Since the presence of warm dust indicates that collisions are taking place between rocks in the protoplanetary disk it is not unreasonable to suppose that the collisions would begin earlier and be more numerous and intense in a massive disk. By the time that process is complete, collisions in thinner disks would only be beginning, producing their own signature of warm dust.
If indeed there is little overlap between warm dust stars in the different age groups, then the proportion of stars forming rocky planets could be much higher – as high, in fact, as 60%! This is a remarkable number and it suggests rocky planets will be found in large numbers once the technology to detect these small objects is perfected. But even the lower estimate of 20% is still substantially higher than the 6-12% frequency of gas giant planets detected to date around distant stars. "The correct answer probably lies somewhere between the pessimistic case of less than 20% and the optimistic case of more than 60%" said Meyer. If Meyer and his colleagues are correct, then rocky planets may be plentiful indeed in the galaxy. And if terrestrial planets are common, wouldn't it be reasonable to expect that a substantial number of them are of Earth-like dimensions? And wouldn't some of these planets orbit within the "habitable zone", where water can exist in liquid form? Isn't it likely, in other words, that numerous alien "Earths" are out there, biding their time until our detection methods to become sensitive enough to find them? This study suggests that they are. But until we detect another "Earth" orbiting a distant and alien Sun, we will never know for sure. Did you like this article? Send
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