Ongoing comet massacre around bright star Fomalhaut

Each day, the equivalent of either two 10 kilometer-sized comets – or 2,000 1 kilometer-sized comets – might be crushed into small fluffy, dust particles orbiting Fomalhaut.

The bright star Fomalhaut – approximately 25 light-years away in the constellation Piscis Austrinus the Southern Fish – might be the site of collisions that destroy up to thousands of icy comets every day, astronomers announced today (April 11, 2012). These astronomers reached this conclusion after studying the dusty belt around the Fomalhaut with ESA’s Herschel Space Observatory. These astronomers also confirmed the existence of a possible planet, Fomalhaut b, in orbit around this star.

Fomalhaut is believed to be a young star, only 100 to 300 million years old. It is about twice as massive as our sun, with a subsequently shorter lifespan of only about a billion years. In some ways, studies of this star are like glimpsing back into our solar system’s early history.

Dust belt surrounding bright star Fomalhaut, as seen in the far-infrared by the Herschel Space Observatory. These new images of this dust belt show the belt in more detail at far-infrared wavelengths than ever before. Image Credit: ESA

See the bright spot in the image above, on one side of the belt? Fomalhaut is slightly off-center and closer to the southern side of the belt, and so the southern side is warmer and brighter than the northern side. Astronomer Bram Acke at the University of Leuven in Belgium and his colleagues reached this conclusion, when they analysed temperatures in Fomalhaut’s dust belt via the Herschel Space Observatory data. They found temperatures in Fomalhaut’s belt of dust to be between -230 and –170ºC.

The Herschel temperature data also reveal the possibility of an ongoing comet massacre in this young solar system. Temperatures are consistent with the presence of small solid particles in Fomalhaut’s dust belt, with the sizes of the particles of only a few millionths of a meter across. But, according to Acke and colleagues, this conclusion was in conflict with earlier Hubble Space Telescope observations, which had suggested solid grains more than ten times larger. The resolution to the paradox led to the idea of a comet massacre going on now around Fomalhaut. According to a press release:

Those observations collected starlight scattering off the grains in the belt and showed it to be very faint at Hubble’s visible wavelengths, suggesting that the dust particles are relatively large. But that appears to be incompatible with the temperature of the belt as measured by Herschel in the far-infrared.

To resolve the paradox, Dr Acke and colleagues suggest that the dust grains must be large fluffy aggregates, similar to dust particles released from comets in our own solar system.

These would have both the correct thermal and scattering properties. However, this leads to another problem.

The bright starlight from Fomalhaut should blow small dust particles out of the belt very rapidly, yet such grains appear to remain abundant there.

The only way to overcome this contradiction is to resupply the belt through continuous collisions between larger objects in orbit around Fomalhaut, creating new dust.

To sustain the belt, the rate of collisions must be impressive: each day, the equivalent of either two 10 kilometer-sized comets or 2,000 1 kilometer-sized comets might be crushed into small fluffy, dust particles orbiting Fomalhaut.

These scientists added that, to keep the collision rate so high, there must be between 260 billion and 83 trillion comets in the belt, depending on their size. Our own solar system is thought to have a similar number of comets in its Oort Cloud, which formed from objects scattered from a disk surrounding the sun when it was young, as Fomalhaut is now.

The Hubble Space Telescope took images like this one of Fomalhaut in the early part of this century. This image shows the possible location of the unconfirmed planet Fomalhaut b in 2004 and 2006. The more recent Herschel Space Observatory image confirms the possibility of this planet. Image Credit: NASA, ESA, P. Kalas, J. Graham, E. Chiang, E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center), M. Fitzgerald (Lawrence Livermore National Laboratory), and K. Stapelfeldt and J. Krist (NASA Jet Propulsion Laboratory)

Click here to expand this image

The image above is a composite Hubble Space Telescope image from 2004 and 2006. It shows the movement of a possible planet, Fomalhaut b, in orbit around Fomalhaut. The new Herschel images of Fomalhaut’s dusty disk confirm the possible presence of this planet. Both the narrowness and asymmetry of the Herschel images are thought to be due to the gravity of the planet.

I remember the excitement generated in the 1980s when a dust belt was first discovered by the IRAS satellite to be orbiting Fomalhaut. This discovery was prior to the discovery of any planets orbiting other stars (there are 763 known extrasolar planets now, as of April 8, 2012). The dust belt around Fomalhaut was taken as a possible sign of a planetary system in formation when it was found in the 1980s. It was one of the first pieces of actual evidence that planetary systems do indeed exist.

If the new planet is confirmed, Fomalhaut will be the third-brightest star thought to have an orbiting planet, after the star Pollux in the constellation Gemini and our own sun. That will be exciting, because Fomalhaut is such a prominent and beloved star. It is visible to earthly observers in the Northern Hemisphere in our autumn months, when it appears as the only bright star on an otherwise empty-looking expanse of night sky (not truly empty, of course, but only lacking in other bright stars). Fomalhaut is sometimes called the “autumn star” – or the “loneliest star” because of its solitary appearance. If its planet is confirmed, it will be less lonely, and we on Earth will be able to gaze up at this bright star and imagine its orbiting world.

An artist’s impression of Fomalhaut b, a planet found in orbit around the star Fomalhaut in 2008. Image Credit: ESA, NASA, and L. Calcada

Bottom line: Astronomers examining temperature data from the orbiting Herschel Space Observatory now believe there is a “comet massacre” going on around Fomalhaut, which is one of our sky’s brightest stars. The Herschel Space Observatory images in the far infrared reveal more detail about Fomalhaut’s orbiting ring of dust than ever before, and they also suggest the cometary collision as well as provide support for a possible planet, Fomalhaut b, in orbit around this star.

Fomalhaut: First visible extrasolar planet

Deborah Byrd