Astronomers Watch as a Black Hole Eats a Rogue Planet

In Star Wars, the Millennium Falcon narrowly escaped being devoured by an exogorth (space slug) slumbering inside an asteroid crater. An unsuspecting rogue giant planet wasn’t as lucky. Astronomers using the Integral space observatory were able to watch as the planet was eaten by a black hole that had been inactive for decades. It woke up just in time to make a meal out of the unwary planet.

“The observation was completely unexpected, from a galaxy that has been quiet for at least 20–30 years,” says Marek Nikolajuk of the University of Bialystok, Poland, lead author of the paper in Astronomy & Astrophysics.

Nikolajuk and his team added that the event is a preview of a similar feeding event that is expected to take place with the black hole at the center of our own Milky Way Galaxy.

The discovery in galaxy NGC 4845, 47 million light-years away, was made by Integral, with follow-up observations from ESA’s XMM-Newton, NASA’s Swift and Japan’s MAXI X-ray monitor on the International Space Station.

Astronomers were using Integral to study a different galaxy when they noticed a bright X-ray flare coming from another location in the same wide field-of-view. Using XMM-Newton, the origin was confirmed as NGC 4845, a galaxy never before detected at high energies.

Along with Swift and MAXI, the emission was traced from its maximum in January 2011, when the galaxy brightened by a factor of a thousand, and then as it subsided over the course of the year.

By analyzing the characteristics of the flare, the astronomers could determine that the emission came from a halo of material around the galaxy’s central black hole as it tore apart and fed on an object of 14–30 Jupiter masses, and so the astronomers say the object was either a super-Jupiter or a brown dwarf.

This object appears to have been ‘wandering,’ which would fit the description of recent studies that have suggested that free-floating planetary-mass objects of this kind may occur in large numbers in galaxies, ejected from their parent solar systems by gravitational interactions.

The black hole in the center of NGC 4845 is estimated to have a mass of around 300,000 times that of our own Sun. The astronomers said it also appears to enjoy playing with its food: the way the emission brightened and decayed shows there was a delay of 2–3 months between the object being disrupted and the heating of the debris in the vicinity of the black hole.

“This is the first time where we have seen the disruption of a substellar object by a black hole,” said co-author Roland Walter of the Observatory of Geneva, Switzerland. “We estimate that only its external layers were eaten by the black hole, amounting to about 10% of the object’s total mass, and that a denser core has been left orbiting the black hole.”

The flaring event in NGC 4845 might be similar to what is expected to happen with the supermassive black hole at the center of our own Milky Way Galaxy, perhaps even this year, when an approaching Earth-mass gas cloud is expected to meet its demise.

Along with the object seen being eaten by the black hole in NGC 4845, these events will tell astronomers more about what happens to the demise of different types of objects as they encounter black holes of varying sizes.

“Estimates are that events like these may be detectable every few years in galaxies around us, and if we spot them, Integral, along with other high-energy space observatories, will be able to watch them play out just as it did with NGC 4845,” said Christoph Winkler, ESA’s Integral project scientist.

The team’s paper: Tidal disruption of a super-Jupiter in NGC 4845

Source: ESA

New Rogue Planet Found, Closest to our Solar System

This artist’s impression shows the free-floating planet CFBDSIR J214947.2-040308.9. Credit: ESO/L. Calçada/P. Delorme/Nick Risinger/R. Saito/VVV Consortium

Rogue planets – also known as free floating planets – are pretty intriguing. They are not orbiting a star but instead are wandering through the galaxy, having been either forcibly ejected from a solar system or having formed very early on in the Universe. While only a handful of these planets have been actually found, astronomers estimate these vagrant worlds could vastly outnumber stars. In fact, it’s been suggested there could be 100,000 times more rogue planets than stars in our Milky Way galaxy alone!

The latest rogue world to be found is exciting in that it is the closest such object to our Solar System so far. At a distance of about 100 light-years, its comparative proximity, along with the absence of a bright star very close to it, has allowed the team to study its atmosphere in great detail. Astronomers say this object gives them a preview of the exoplanets that future instruments will be able to find – and potentially take image of — around stars other than the Sun. But the planet also seems to be loosely tied to a roving group of stars, called the AB Doradus Moving Group.

The new rogue planet, with the ungainly name of CFBDSIR J214947.2-040308.9 (CFBDSIR2149 for short), was found using the Very Large Telescope and the Canada-France-Hawaii Telescope. The astronomers, led by Philippe Delorme from the Institut de planétologie et d’astrophysique de Grenoble, CNRS/Université Joseph Fourier, France, are calling the object a rogue planet candidate for now, as they want to study it further to confirm its free-floating status.

Moving star systems are equally intriguing. The AB Doradus Moving Group is the closest such group to our Solar System, and the stars drift through space together in a pack. They are thought to have formed at the same time. If the new rogue planet actually is associated with this moving group, astronomers say it will be possible to deduce much more about it, including its temperature, mass, and what its atmosphere is made of. There remains a small probability that the association with the moving group is by chance.

The link between the new object and the moving group is the vital clue that allows astronomers to find the age of the newly discovered object. Without knowing its age, it’s not possible to know whether it is really a planet, or a brown dwarf, a “failed” star that lack the bulk to trigger the reactions that make stars shine.

This is the first isolated planetary mass object ever identified in a moving group, and the association with this group makes it the most interesting free-floating planet candidate identified so far.

This closeup of an image captured by the SOFI instrument on ESO’s New Technology Telescope at the La Silla Observatory shows the free-floating planet CFBDSIR J214947.2-040308.9 in infrared light. This object, which appears as a faint blue dot at the centre of the picture, is the closest such object to the Solar System. Credit: ESO/P. Delorme.

“Looking for planets around their stars is akin to studying a firefly sitting one centimetre away from a distant, powerful car headlight,” said Delorme. “This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up.”

Free-floating objects like CFBDSIR2149 are thought to form either as normal planets that have been booted out of their home systems, or as lone objects like the smallest stars or brown dwarfs. In either case these objects are intriguing — either as planets without stars, or as the tiniest possible objects in a range spanning from the most massive stars to the smallest brown dwarfs.

“These objects are important, as they can either help us understand more about how planets may be ejected from planetary systems, or how very light objects can arise from the star formation process,” says Philippe Delorme. “If this little object is a planet that has been ejected from its native system, it conjures up the striking image of orphaned worlds, drifting in the emptiness of space.”

If CFBDSIR2149 is not associated with the AB Doradus Moving Group, the astronomers say it is trickier to be sure of its nature and properties, and it may instead be characterized as a small brown dwarf. Both scenarios represent important questions about how planets and stars form and behave.

“Further work should confirm CFBDSIR2149 as a free-floating planet,” said Delorme. “This object could be used as a benchmark for understanding the physics of any similar exoplanets that are discovered by future special high-contrast imaging systems, including the SPHERE instrument that will be installed on the VLT.”

This video shows an artist’s impression of the free-floating planet CFBDSIR J214947.2-040308.9. In the first part of the sequence the planet appears as a dark disc in visible light, silhouetted against the star clouds of the Milky Way. This is the closest such object to the Solar System and the most exciting candidate free-floating planet found so far. It does not orbit a star and hence does not shine by reflected light; the faint glow it emits can only be detected in infrared light. In the final sequence we see an infrared view of the object with the central parts of the Milky Way as seen by the VISTA infrared survey telescope as background. The object appears blueish in this near-infrared view because much of the light at longer infrared wavelengths is absorbed by methane and other molecules in the planet’s atmosphere. In visible light the object is so cool that it would only shine dimly with a deep red colour when seen close-up.

Read the team’s research paper here (pdf).

Source: ESO