Centauri Dreams sometimes gets e-mail from readers asking how research results can be so contradictory. We’ve discussed gas giants around red dwarf stars, for example, noting theories that such planets are rare in this environment. And then we come up with stars like Gliese 876 and GJ 317, both red dwarfs, and both sporting not one but two gas giants as companions. But stand by, for in a moment we’ll look at new evidence that outer gas giants are indeed rare, and not just around M dwarfs.
What’s going on? The answer is that exoplanetary studies are a work in progress, and will continue to be as far into the future as I can see. We have identified over 200 exoplanets in a galaxy of several hundred billion stars. You bet we’re going to find anomalous situations that challenge every theory we have. And the idea is to put hard scientific work out there for review and critique, noting methodologies and explaining conclusions, thus letting other scientists have a go at the same data.
Those seeking conclusive answers this early in the game are going to find this frustrating, but that’s how science works, and it’s a measure of the complexity of what we’re studying that exoplanetary systems yield their secrets only slowly and over time. The new work for today is an example, a collaboration between US and European astronomers that surveyed 54 young, nearby stars thought to be candidates for Jupiter-class planets at distances beyond Jupiter’s own 5 AU from their star.
Radial velocity techniques are great for finding planets close to the stars they orbit, but much more problematic when dealing with outer planets. So the survey team worked with direct-imaging methods instead, and methane-sensitive imagers specifically designed for this operation. Their conclusion is a bit startling: The survey failed to find a single extrasolar planet in the outer parts of any of the nearby systems it studied. Says graduate student Eric Nielsen (Seward Observatory), “There is no ‘planet oasis’ between 20 and 100 AU. We achieved contrasts high enough to find these super Jupiters, but didn’t.”
One thing scientists will now look at as they probe this work is the imaging technique involved in the survey, based on an instrument called the Simultaneous Differential Imager (SDI) that has been used with both the ESO Very Large Telescope 8.2-meter instrument in Chile and the 6.5-meter telescope at the UA/Smithsonian MMT Observatory on Mount Hopkins, Arizona. The SDI camera splits the light from a single object into four images, which are then sent through methane-sensitive filters to a detector array. Ideally, the bright star disappears while the methane-laden companion comes into view, as shown below.
The method has had success in the past, discovering a brown dwarf around the star SCR 1845-6357, some 12.7 light years away. And Laird Close (University of Arizona), one of the developers of the SDI, finds it powerful enough to say this: “We certainly had the ability to detect outer super Jupiter planets at 10 AU, and farther out, around young sun-like stars.”
Image: Comparison of images taken with SDI on and off. A number of fake planets (at separations of 0.55″, 0.85″, and 1.15″ from the star) were added in to this data, which was then analyzed first using the SDI method and second, using standard adaptive optics techniques. The simulated planets, each seen as a pair of black-and-white dots 33 degrees apart in the SDI image, are easily detected yet are 10,000 times fainter than the central star in the standard adaptive optics analysis. Credit: Laird Close/University of Arizona.
But they didn’t. What would be the constraints on outer gas giant formation around these stars, and how does the survey result affect our current notions of planet formation? Work like this is interesting precisely because it targets filling in the gaps in our knowledge of outer exoplanetary systems, helping us ultimately to learn whether our own Solar System is somewhat average or a departure from the norm. And the answers to the questions it raises will be worked out over time and with the contribution of further surveys using a wide variety of technologies.
Surprises, then, are the nature of the game, and should be considered as opportunities to refine existing theories or suggest new ones. We’ll all watch this process at work as researchers study the two papers involved. They’re Biller et al., “An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with SDI at the VLT and MMT,” accepted by the Astrophysical Journal (abstract available) and Nielsen et al., “Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations,” submitted to the Astrophysical Journal (abstract).