This visible-light image from the Hubble Space Telescope shows the vicinity of the star Fomalhaut, including the location of its dust ring and disputed planet, Fomalhaut b. A coronagraphic mask helped dim the star’s brightness. This view combines two 2006 observations that were taken with masks of different sizes (1.8 and 3 arcseconds). (Credit: NASA/ESA/T. Currie, U. Toronto)
Just in time for Halloween, astronomers are bringing an extrasolar planet back from the dead. Another look at the nearby star Fomalhaut reveals that a planet, named Fomalhaut b, is actually, really there, refuting a previous claim against its existence. In 2008, it was announced that a large, Saturn mass planet shepherded a large dust ring and was spotted in visual images from Hubble, and was said to be the first exoplanet ever directly imaged in visible light around another star. But in late 2011 infrared observations called the previous detections into question. A new analysis of data from Hubble, however, brings the planet conclusion back to life.
It’s like a zombie planet that just won’t die.
Fomalhaut is the brightest star in the constellation Piscis Austrinus and lies 25 light-years away.Originally, Fomalhaut b was estimated to be approximately the size of Saturn, and might even have rings. It resides within a debris ring which encircles the star Fomalhaut, located about 25 light-years away from Earth.
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Then, later studies claimed that this planetary interpretation is incorrect. Based on the object’s apparent motion and the lack of an infrared detection by NASA’s Spitzer Space Telescope, they argued that the object is a short-lived dust cloud unrelated to any planet.
But still another observation brings this planet back.
“Although our results seriously challenge the original discovery paper, they do so in a way that actually makes the object’s interpretation much cleaner and leaves intact the core conclusion, that Fomalhaut b is indeed a massive planet,” said Thayne Currie, an astronomer formerly at NASA’s Goddard Space Flight Center in Greenbelt, Md., and now at the University of Toronto.
The discovery study reported that Fomalhaut b’s brightness varied by about a factor of two and cited this as evidence that the planet was accreting gas. Follow-up studies then interpreted this variability as evidence that the object actually was a transient dust cloud instead.
In the new study, Currie and his team reanalyzed Hubble observations of the star from 2004 and 2006. They easily recovered the planet in observations taken at visible wavelengths near 600 and 800 nanometers, and made a new detection in violet light near 400 nanometers. In contrast to the earlier research, the team found that the planet remained at constant brightness.
The team attempted to detect Fomalhaut b in the infrared using the Subaru Telescope in Hawaii, but was unable to do so. The non-detections with Subaru and Spitzer imply that Fomalhaut b must have less than twice the mass of Jupiter.
Another contentious issue has been the object’s orbit. If Fomalhaut b is responsible for the ring’s offset and sharp interior edge, then it must follow an orbit aligned with the ring and must now be moving at its slowest speed. The speed implied by the original study appeared to be too fast. Additionally, some researchers argued that Fomalhaut b follows a tilted orbit that passes through the ring plane.
Using the Hubble data, Currie’s team established that Fomalhaut b is moving with a speed and direction consistent with the original idea that the planet’s gravity is modifying the ring.
“What we’ve seen from our analysis is that the object’s minimum distance from the disk has hardly changed at all in two years, which is a good sign that it’s in a nice ring-sculpting orbit,” explained Timothy Rodigas, a graduate student in the University of Arizona and a member of the team.
Currie’s team also addressed studies that interpret Fomalhaut b as a compact dust cloud not gravitationally bound to a planet. Near Fomalhaut’s ring, orbital dynamics would spread out or completely dissipate such a cloud in as little as 60,000 years. The dust grains experience additional forces, which operate on much faster timescales, as they interact with the star’s light.
“Given what we know about the behavior of dust and the environment where the planet is located, we think that we’re seeing a planetary object that is completely embedded in dust rather than a free-floating dust cloud,” said team member John Debes, an astronomer at the Space Telescope Science Institute in Baltimore, Md.
A paper describing the findings has been accepted for publication in The Astrophysical Journal Letters.
And as we reported in April, another team using the Atacama Large Millimeter/sub-millimeter Array (ALMA) indicated they found evidence of Fomalhaut b, and maybe even more planets in the system, giving more credence to the planet’s exitence.
Because astronomers detect Fomalhaut b by the light of surrounding dust and not by light or heat emitted by its atmosphere, it no longer ranks as a “directly imaged exoplanet.” But because it’s the right mass and in the right place to sculpt the ring, Currie’s team thinks it should be considered a “planet identified from direct imaging.”
Fomalhaut was targeted with Hubble most recently in May by another team. Those observations are currently under scientific analysis and are expected to be published soon.