Exoplanet That Was Previously Discovered And Then Discounted Turns Out To Be Real

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A previously discovered exoplanet, that had its existence called into question by outside researchers, is most likely a real planet, according to new research.

The work, done with NASA’s Hubble Space Telescope, is reviving the idea that the nearby star Fomalhaut is orbited by a massive exoplanet. The research suggests that the planet “is a rare and possibly unique object that is completely shrouded by dust.”

The star Fomalhaut is located in the constellation Piscis Austrinus 25 light-years away, and is its brightest star as observed from the Earth.

“In November 2008, Hubble astronomers announced the exoplanet, named Fomalhaut b, as the first one ever directly imaged in visible light around another star. The object was imaged just inside a vast ring of debris surrounding but offset from the host star. The planet’s location and mass — no more than three times Jupiter’s — seemed just right for its gravity to explain the ring’s appearance.”

That report was followed by other studies that said that that interpretation, as a planetary body, wasn’t accurate. “Based on the object’s apparent motion and the lack of an infrared detection by NASA’s Spitzer Space Telescope, they argue that the object is a short-lived dust cloud unrelated to any planet.”

The new research is supporting the idea that it is, in fact, definitely a planet.

“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 researchers tried to detect Fomalhaut b in the infrared spectrum by using the Subaru Telescope in Hawaii, but weren’t successful. “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.”

Making use of the Hubble data, the researchers clarified 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.

The researchers 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.

“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 is also the subject of another recent study done with the Hubble Space Telescope. The results of those observations are expected to be published in the near future.

The research was just published in The Astrophysical Journal Letters.

Source: NASA/Goddard Space Flight Center

Image Credits: NASA/ESA/T. Currie, U. Toronto; ESA; Hubble, M. Kornmesser; and ESO, L. Calçada and L. L. Christensen

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