One of Hubble’s most celebrated discoveries is the November 2008 announcement of the first visible-light image of a planet orbiting another star. Along with several infrared observations announced by other observatories, that month marked the historic beginning of direct detections of extrasolar planets. Previous discoveries were indirect, deduced from the way the gravity, size, or motion of the planet affects the light from its host (or another) star. Since then, we’ve also been able to see infrared images of a few planets in other solar systems.
Hubble’s discovery, however, has been literally clouded from the start. The star Fomalhaut is 25 light-years away from Earth. The planet, dubbed “Fomalhaut b,” has an orbit more than 100 times larger than Earth’s orbit, and a size estimated to be about three times the mass of Jupiter. Considering the star’s brightness and distance, as well as the planet’s orbit and size, the object Hubble sees is too bright to be just the planet. The explanation has been that a huge ring of ice and dust around the planet is reflecting the extra starlight.
Observers using the Spitzer Space Telescope have recently questioned that explanation. As reported by Universe Today, a paper will appear in the “Astrophysicial Journal” that argues a better interpretation of the image is a “transient or semi-transient dust cloud.” This conclusion is based on infrared observations that fail to show the expected emission from a planet. Because planets are brighter at infrared wavelengths, the astronomers concluded that the visible-light observation is only from dust and not from a planet. While the gravitational argument for a planet remains solid, the research team found that the observed light source is “highly unlikely” to be a giant planet.
As one might expect, the original researchers disagree with these findings. They considered the dust cloud idea, but ruled it out. In their opinion, the resolution and sensitivity of Spitzer is too low to warrant such conclusions, and their postulated ring around the planet fits with the infrared observations.
One could promote this as the NASA Great Observatories in a scientific smackdown. Hubble punches with a planet. Spitzer counterattacks with a dust cloud. It’s an astronomical battle royale for extrasolar supremacy! But that would be silly.
This discussion is just the natural progress of science. Results, especially the really significant ones, must be checked and re-checked by many astronomers at many observatories before they become fully accepted. The original paper clearly identified the discrepancy of the extra light observed. The follow-up paper does a great service for everyone by providing more data to examine. Neither explanation is completely convincing at this time.
Eventually, more data will reveal what is truly being observed. In particular, the James Webb Space Telescope has the resolution of Hubble in the infrared wavelengths of Spitzer. JWST is slated for launch in 2018, and its observations should be able to distinguish between the two hypotheses. Or, more dramatically, JWST will be the ultimate referee of this cosmic clash of telescopic titans.