Revisiting the exoplanet puzzle

ASTRONOMY

Is Fomalhaut B a star or a planet? That’s the question that has been on many planet hunters’ minds since the mid-80s. The latest round of Hubble Space Telescope observations could help resolve the controversy, writes Roy Cowen 

Twinkle, twinkle, little spot: Are you a real world or not? Pinning down the elusive nature of Fomalhaut b, a faint speck of light discerned within the glare of a nearby star, has proved to be far from child’s play. At stake are bragging rights to the first optical image of an exoplanet, and one that is close enough to be studied in detail. But since Fomalhaut b was first reported in 2008, its identity has been the subject of intense debate. The latest round of Hubble Space Telescope observations, that began on May 27, could help to resolve the controversy.

Located a mere 7.7 parsecs (25 light years) away, the star Fomalhaut has captivated planet hunters since the mid-1980s, when infrared observations revealed that it is surrounded by dust – a possible signal of a recently formed solar system. In 2005, after further study showed that the dust took the form of a narrow band with a sharp inner edge, astronomer Paul Kalas of the University of California, Berkeley, and his colleagues suggested that a planet’s gravity was shaping the dust’s structure.

Planet or star?

Proof seemed to come in 2008, when Kalas and his collaborators reported that Hubble observations of the Fomalhaut star system nearly two years apart revealed a spot that shifted position, as would be expected of a body orbiting just inside the dust band’s inner edge. From its brightness, they proposed that the object, Fomalhaut b, is a planet roughly half the mass of Jupiter. That would make it the first exoplanet to be directly imaged in visible light rather than through indirect effects, such as a wobble of the parent star. 

Because of the star’s extreme brilliance, says Kalas, “Everything you see, even the dust belt, is extremely difficult to image.” Puzzling details in the data left the matter far from settled. By the time of the second Hubble observation, in 2006, Fomalhaut b had faded to half the brightness observed in 2004 at an optical wavelength of 0.6 micrometers. Even more troubling, a preliminary search for the object in the infrared – the part of the spectrum in which newborn planets are expected to glow the brightest – failed to reveal anything.

Efforts to learn more were thwarted in January 2007, when the camera channel used by Hubble to acquire the image stopped working. Instead, in 2010, Kalas made a third set of observations using the older Space Telescope Imaging Spectrograph on Hubble. Those results suggested that the object is heading into the dust band – not the path expected for a planet sculpting the band’s inner edge. Earlier this year, another team reported that NASA’s infrared Spitzer Space Telescope had failed to detect the body at all. 

What dust suggests

On April 27, a modelling study showed that a star might form a narrow band of dust without needing a planet to shape it. “There could still be a planet there, but the light detected has to come from something else, most likely from scattering of starlight by a large cloud of dust,” says astronomer David Lafreniere of the University of Montreal in Quebec, Canada, who is a co-author of the Spitzer study. 

Lafreniere notes that another recent study supports that idea. Images from the European Space Agency’s Herschel Space Observatory reveal that the band around Fomalhaut is packed with fine dust, which the star’s radiation pressure continually sweeps into interstellar space. For the dust band to be maintained, the study’s authors conclude, the equivalent of 2,000 one-kilometer-wide comets must be destroyed in the ring every day. Such destruction could create a compact cloud of dust that would shine like the object seen by Hubble, says Lafreniere. Adding to the intrigue, Christian Marois and Rapha Galicher of the Herzberg Institute for Astrophysics near Victoria in British Columbia, Canada, have reanalysed the original Hubble data and say they cannot confirm that Fomalhaut b has faded or that it is veering into the dust band. Marois adds that a much bigger surprise could be lurking in the analysis, but he won’t unveil details until mid-June, when Kalas plans to release the results of the next Hubble observations. 

Those spectrograph images should help to pin down the orbit of Fomalhaut b and whether it varies in brightness, Kalas says. But to determine whether the object is truly a planet, he adds, Hubble will have to monitor it for several more years to see whether it behaves as a planet, a dust cloud or something else.Some researchers, however, have already consigned Fomalhaut b to the dustbin. “Some people are bending over backward to keep the planet alive. There’s no justification any more,” says Ray Jayawardhana, an astronomer at the University of Toronto in Ontario, Canada. Along with Lafreniere and other collaborators, he is co-discoverer of another object, J1609, which he says is the first true exoplanet to be imaged directly.Over to you, Hubble.