Right now is a genuinely exciting time to be studying exoplanets. With instruments like NASA’s Kepler space telescope staring unblinkingly into the sky, the discoveries are coming thick and fast. As I type this, we know of 859 confirmed exoplanets in 676 star systems, with literally thousands more exoplanet candidates awaiting confirmation.
The past year has seen announcements of some of the most Earth-like planets yet discovered, and even a small planet around one of the two stars in Alpha Centauri, our nearest neighboring star system. But in all of this, there’s been one thing in the back of our minds that has been sitting slightly uncomfortably: 25 light-years away lies Fomalhaut b, a planet that has been the cause of an astronomical altercation.
If you’ve been reading the right news feeds, you’ll probably know by now that Fomalhaut b has officially been confirmed as a planet. And it’s an interesting one, too. Orbiting the star Fomalhaut, an A-type star somewhat more massive and hotter than the sun, this planet has a wildly eccentric orbit. As befitting a larger star, Fomalhaut b’s orbit is huge compared to our solar system.
Even at the closest point in its orbit, the orbits of every planet in our own solar system could fit between it and Fomalhaut (as shown in this, now outdated, image). At the furthest point in its orbit, it reaches nearly 10 times the distance between Neptune and the sun, ploughing through a thick disk of dusty material held in Fomalhaut’s gravitational grip.
So what was the controversy over this planet? As it happens, it’s quite an interesting story, and an excellent example of the way good science works.
You see, Fomalhaut b was, for a long time, the poster child of exoplanet discovery. Its presence was inferred from some Hubble observations taken in 2005. A debris disk like the one encircling Fomalhaut, still quite a young star, is not unusual in itself. The strange thing about this disk is that it has a sharp inner boundary, as if some massive object had cleared away all of the dust from the inner part of the disk.
It wouldn’t be until 2008 that two astronomers at UC Berkeley, James Graham and Paul Kalas, were re-examining Hubble images taken in 2004 and 2006. With a rush of excitement, they realized that a small cluster of pixels in the image had moved, and the only explanation was that they were looking at the first ever direct image of a planet orbiting another star.
Kalas and Graham’s images showed the planet in scattered starlight at 600 nm, and thermal emission at 800 nm. However, Fomalhaut b was destined to cause some controversy amongst the exoplanet community. As any good scientist should, others set out to try and detect the planet for themselves. Unfortunately, that caused some problems — despite looking carefully, no trace of any planet was found where they were expecting to see it.
In particular, attempts to take infrared images of it were met with failure. Puzzling, because any sizable planet should shine brightly, and obviously, in infrared light. Coupled with the fact that, from the previous work, it seemed that this object must be moving too fast to have the predicted effect on Fomalhaut’s debris disk, some began to doubt that what they were seeing was a planet at all.
In 2011, researchers at the Atacama Large Millimetre Array (ALMA) devised a hypothesis under which Fomalhaut b wasn’t actually needed to explain the star’s debris disk. With some convincing evidence, many had to agree that things didn’t look good for the would-be planet.
So it would remain, until last year, when Thayne Currie at the University of Toronto finally made an independent confirmation of the planet, presenting new observations, re-examining old observations, and giving a brand new detection at a new and much bluer wavelength of 400 nm. After showing that Fomalhaut b was actually quite unlikely to be observed in infrared, and recalculating some details about it, Fomalhaut b was once again starting to win people over.
Finally, a third group of researchers, Raphael Galicher and Christian Marois, independently detected and confirmed it. This was indeed a real planet we were all looking at.
Galicher and Marois could tell, this was a rather unusual planet they were looking at. Its spectrum of light suggests that it lies shrouded in dust, perhaps a huge ring system or the aftermath of a recent collision between two dwarf planets. The latter idea would nicely explain a narrow and suspiciously new outer disk seen around Fomalhaut.
If current calculations are proven to be correct, Fomalhaut b is due to dive back into that debris disk around 2032, at which point astronomers will start watching very carefully to see if they can spot any collisions with further objects. If the planet orbits in the same plane as the debris disk, such collisions are entirely possible.
Weighing in at an estimated 3 Jupiter masses, Fomalhaut b is no lightweight. With it’s potential for destroying any dwarf planets that may lie in its path, it is perhaps fitting that this macabre specter of a planet be given the nickname of the “zombie planet” — after having been effectively resurrected from the academic dead!
Image credit: NASA, ESA, and P. Kalas (University of California, Berkeley)