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Several Billion Jupiter-sized Rogue Planets May be Roaming Freely in the Our Galaxy Without a Host Star
Artist's impression of a free-floating planet lost in deep space. A team led by astronomers of the Laboratoire d’astrophysique de Bordeaux has reported the discovery of about 100 free-floating planets roaming in our galaxy. Rogue planets are elusive cosmic objects that have masses comparable to those of the planets in our Solar System but do not orbit a star, instead roaming freely on their own. Not many were known until now, but a team of astronomers, using data from several European Southern Observatory (ESO) telescopes and other facilities, have just discovered about 100 new rogue planets in our galaxy. This is the largest group of rogue planets ever discovered -- an important step towards understanding the origins and features of these mysterious galactic nomads. (Image Credit: University of Bordeaux)
Several Billion Jupiter-sized Rogue Planets May be Roaming Freely in the Our Galaxy Without a Host Star
Rogue planets are elusive cosmic objects that have masses comparable to those of the planets in our Solar System but do not orbit a star, instead roaming freely on their own. Not many were known until now, but the astronomers of the European project COSMIC DANCE, led by Herve Bouy, a professor of the University of Bordeaux in France, at the Laboratoire d’astrophysique de Bordeaux (LAB – CNRS and University of Bordeaux), have just discovered between 70 and 170 new rogue planets in the Milky Way. This is the largest collection of rogue planets ever discovered at one time and an important step towards understanding the origins and features of these mysterious galactic nomads.
Possible examples of rogue planets have been found before, but without knowing their ages, it was not possible for astronomers to know whether they were really planets or brown dwarfs - failed stars that lack the bulk to trigger the reactions that make stars shine. These objects started to become known in the 1990s when astronomers found that the point at which a brown dwarf crosses over into the planetary mass range was difficult to determine.
“We did not know how many to expect and are excited to have found so many,” says Nuria Miret-Roig, a postdoctoral researcher at the University of Vienna, Austria, who received her PhD from the University of Bordeaux.
Wanderers in the Galaxy
Rogue planets, lurking far away from any star illuminating them, would normally be impossible to image. However, Nuria Miret-Roig and her team took advantage of the fact that, in the few million years after their formation, these planets are still hot enough to glow, making them directly detectable by sensitive cameras on large telescopes. They found at least 70 new rogue planets with masses comparable to Jupiter’s in a star-forming region close to our Sun, located within the Scorpius and Ophiuchus constellations.
To spot so many rogue planets, the astronomers used data spanning about 20 years from several telescopes on the ground and in space. “We measured the tiny motions, the colors and luminosities of tens of millions of sources in a large area of the sky,” explains Nuria Miret-Roig. “These measurements allowed us to securely identify the faintest objects in this region, the rogue planets.”
The team of the COSMIC DANCE project used observations from several European Southern Observatory (ESO) telescopes located in Chile, along with NSF’s NOIRLab facilities, the Canada France Hawaii telescope and the Subaru telescope.
“The vast majority of our data comes from ESO observatories, which were absolutely critical for this study. Their wide field of view and unique sensitivity were keys to our success,” explains Herve Bouy. “We used tens of thousands of wide-field images, corresponding to hundreds of hours of observations, and literally tens of terabytes of data.”
Several Billion Planets Without a Host Star
The team also used data from the European Space Agency’s Gaia satellite, marking a huge success for the collaboration of ground- and space-based telescopes in the exploration and understanding of our Universe.
The study suggests there could be many more of these elusive, starless planets that we have yet to discover. “There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star,” Herve Bouy explains.
By studying the newly found rogue planets, astronomers may find clues to how these mysterious objects form. Some scientists believe rogue planets can form from the collapse of a gas cloud that is too small to lead to the formation of a star, or that they could have been kicked out from their parent system. But which mechanism is more likely remains unknown.
Further advances in technology will be key to unlocking the mystery of these nomadic planets. The team hopes to continue to study them in greater detail with ESO’s forthcoming Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert and due to start observations later this decade. “These objects are extremely faint and little can be done to study them with current facilities,” says Herve Bouy. “The ELT will be absolutely crucial to gathering more information about most of the rogue planets we have found.”
In 2012, Astronomers using ESO's Very Large Telescope and the Canada-France-Hawaii Telescope identified a body that is very probably a planet wandering through space without a parent star. At the time, this was the most exciting free-floating planet candidate so far and the closest such object to the Solar System at a distance of about 100 light-years. Its comparative proximity and the absence of a bright star very close to it, allowed researchers to study its atmosphere in great detail.
The astronomers discovered the object, labelled CFBDSIR2149 that seemed to be part of a nearby stream of young stars known as the AB Doradus Moving Group. The object was identified as part of an infrared extension of the Canada-France Brown Dwarfs Survey (CFBDS), a project hunting for cool brown dwarf stars. Images taken at different times allowed the object's proper motion across the sky to be measured and compared to members of the AB Doradus Moving Group. A detailed study of the object's atmosphere was made using the X-shooter spectrograph on ESO's Very Large Telescope at the Paranal Observatory.
The AB Doradus Moving Group is the closest such group to the Solar System. Its stars drift through space together and are thought to have formed at the same time. If the object is associated with this moving group - and hence it is a young object - it is possible to deduce much more about it, including its temperature, mass, and what its atmosphere is made of.
The association with the AB Doradus Moving Group pinned down the mass of the planet to approximately 4–7 times the mass of Jupiter, with an effective temperature of approximately 430 degrees Celsius. The planet's age would be the same as the moving group itself, that is, 50 to 120 million years.
The link between the rogue planet and the moving group was the vital clue that allows astronomers to find the age of the newly discovered object. This was 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.
The team's statistical analysis of the object's proper motion - its angular change in position across the sky each year - shows an 87 percent probability that the object was associated with the AB Doradus Moving Group and more than 95 percent probability that it was young enough to be of planetary mass, making it much more likely to be a rogue planet rather than a small failed star.
"Looking for planets around their stars is akin to studying a firefly sitting one centimeter away from a distant, powerful car headlight," said Philippe Delorme (Institut de Planetologie et d' Astrophysique de Grenoble, CNRS/Universite Joseph Fourier, France) at the time of the discovery.
"This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up."
"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."
Here’s a Scary Thought -- Hundreds of Rogue Black Holes May Also Roam the Galaxy
It sounds like the plot of a B-rated sci-fi movie -- Rogue black holes roaming our galaxy, threatening to swallow anything that gets too close. Independent calculations in 2009 by Astrophysicists at the Harvard-Smithsonian Center for Astrophysics and Vanderbilt University suggested that hundreds of massive black holes left over from the galaxy-building days of the early universe may wander the Milky Way. The good news is that they think (and we hope) that the closest rogue black holes reside thousands of light-years away.
“Rogue black holes like this would be very difficult to spot,” said Vanderbilt astronomer Kelly Holley-Bockelmann at the time of the study. Much of this research was done at Penn State in collaboration with Deirdre Shoemaker and Nicolas Yunes of Vanderbilt University before Holley-Bockelmann moved to Vanderbilt. Kayhan Gultekin at the University of Michigan also participated in the study.
“Unless it's swallowing a lot of gas, about the only way to detect the approach of such a black hole would be to observe the way in which its super-strength gravitational field bends the light that passes nearby. This produces an effect called gravitational lensing that would make background stars appear to shift and brighten momentarily,” she says.
The research focused on modeling "intermediate mass" black holes, whose very existence at the time was controversial. Astronomers have ample evidence that small black holes less than 100 solar masses are produced when giant stars explode. There is similar evidence that “super-massive” black holes weighing the equivalent of millions to billions of solar masses sit at the heart of many galaxies, including the Milky Way. In addition, theoreticians predicted that globular clusters – ancient, gravitationally bound groups of 100,000 to a million stars – should contain a third class of black holes, called intermediate mass black holes, that weigh a few thousand solar masses. But so far there have only been two tentative observations of objects of this sort.
In the past, scientists had succeeded in numerically simulating black hole mergers that incorporate Einstein’s theory of relativity. One of the big surprises to come from this effort was the prediction that when two black holes that are rotating at different speeds or are different sizes combine, the newly merged black hole receives a big kick due to conservation of momentum, pushing it away in an arbitrary direction at velocities as high as 4000 kilometers per second.
“This is much higher than anyone predicted. Even the average kick velocity of 200 kilometers per second is extremely high when compared to the escape velocities of typical astronomical objects,” said Holley-Bockelmann. “We realized that basically any black hole merger would kick the new remnant out of a globular cluster, because the escape velocity is less than 100 kilometers per second.”
Holley-Bockelmann’s team ran a number of simulations of the growth of intermediate mass black holes as they combine with a number of stellar-sized black holes, which are plentiful in globular clusters, paying close attention to the kick they received after each merger.
“We used different assumptions for the initial black hole mass, for the range of stellar black hole masses within a globular cluster, and assumed that the spins and spin orientations were distributed randomly. With our most conservative assumptions, we found that, even if every globular cluster started out with an intermediate-sized black hole, only about 30 percent retain them through the merger epoch. With our least conservative assumptions, less than 2 percent of the globular clusters should contain intermediate mass black holes today,” she says.
If the roughly 200 globular clusters in the Milky Way have indeed spawned intermediate-sized black holes, this means that hundreds of them are probably wandering invisibly around the Milky Way, waiting to engulf the nebulae, stars and planets that are unfortunate enough to cross their paths.
Fortunately, the existence of a few rogue black holes in the neighborhood does not present a major danger. “These rogue black holes are extremely unlikely to do any damage to us in the lifetime of the universe,” Holley-Bockelmann stressed. “Their danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!”
Similar work at the time by Ryan O'Leary and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) suggested that hundreds of massive black holes left over from the galaxy-building days of the early universe may also still wander the Milky Way.
"These black holes are relics of the Milky Way's past," said Loeb at the time of the study. "You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the early universe."
According to theory, rogue black holes originally lurked at the centers of tiny, low-mass galaxies. Over billions of years, those dwarf galaxies smashed together to form full-sized galaxies like the Milky Way.
Each time two proto-galaxies with central black holes collided, their black holes merged to form a single, "relic" black hole. During the merger, directional emission of gravitational radiation would cause the black hole to recoil. A typical kick would send the black hole speeding outward fast enough to escape its host dwarf galaxy, but not fast enough to leave the galactic neighborhood completely. As a result, such black holes would still be around today in the outer reaches of the Milky Way halo.
Hundreds of rogue black holes should be traveling the Milky Way's outskirts, each containing the mass of 1000 to 100,000 suns. They would be difficult to spot on their own because a black hole is visible only when it is swallowing, or accreting, matter.
One telltale sign could mark a rogue black hole: a surrounding cluster of stars yanked from the dwarf galaxy when the black hole escaped. Only the stars closest to the black hole would be tugged along, so the cluster would be very compact.
Due to the cluster's small size on the sky, appearing to be a single star, astronomers would have to look for more subtle clues to its existence and origin. For example, its spectrum would show that multiple stars were present, together producing broad spectral lines. The stars in the cluster would be moving rapidly, their paths influenced by the gravity of the black hole.
"The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef," explained O'Leary. "Without the shining stars to guide our way, the black holes would be all but impossible to find."
The number of rogue black holes in our galaxy depends on how many of the proto-galactic building blocks contained black holes at their cores, and how those proto-galaxies merged to form the Milky Way. Finding and studying them will provide new clues about the history of our galaxy.
Locating the star cluster signposts may turn out to be relatively straightforward.
"Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way's halo," said Loeb. "Now that we know what to expect, we can examine existing sky surveys for this new class of objects."
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