Space: The planet that never was

HAVING BEEN in orbit for 30 years now, the Hubble Space Telescope is much lauded for its extraordinary record in probing the Universe from above Earth’s atmosphere. Everything, from nearby planets to the most distant galaxies, has come under the scrutiny of Hubble’s high-resolution cameras.

Some of the telescope’s capabilities are less obvious, however. Not only can it detect visible and ultraviolet light, but it also has significant infrared sensitivity, enabling it to measure the heat radiated by distant objects. Repeated observations of the same object over time also allow astronomers to spot subtle changes.

All these faculties have come together in the strange case of Fomalhaut b, a planet assumed to be in orbit around the Southern Hemisphere star Fomalhaut at a distance of 25 light-years from the Sun.This young star has long been known to have a ring of icy debris around it, suggesting that a planetary system is forming. In 2008 astronomers using the Hubble Telescope announced they had indeed found a planet – the first exoplanet to be observed directly rather than inferred from measurements of its parent star. Observations dating back to 2004 confirmed it was indeed orbiting Fomalhaut.

But there were problems.The proposed Jupiter-like planet should have had a heat signature in the infrared, but none was detected.

Its path through space was also not quite what would be expected from an orbiting planet. In fact, it seemed to be on an escape trajectory from its parent star.There were hints, too, that Fomalhaut b was gradually getting fainter.Then in 2014 the unthinkable happened. New observations indicated the planet had disappeared. It has taken until now for scientists to figure out why.

In work led by András Gáspár of the University of Arizona, researchers have shown that a collision between two icy asteroids perhaps 200km in diameter would produce an expanding cloud of debris that could be mistaken for a planet. It suggests that, following an impact shortly before Hubble’s 2004 observation, the dust cloud slowly grew to be as big as the Earth’s orbit around the Sun, fading to invisibility. Despite their disappointment at the planet’s demise, the astronomers are happy to have observed a cataclysmic event that might occur only once every

200,000 years around a star such as Fomalhaut. A cosmic rarity indeed.

As this artist’s impression shows, Hubble Telescope data reveal an expanding dust cloud from a collision between two large bodies orbiting the bright nearby star Fomalhaut.