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Lost planet, or part of vastest system?
Sept. 5, 2006
Special to World Science
Strange new worlds that redefine the word “planet” are constantly coming to light. But even astronomers, who are used to this, got a jolt six years ago when apparent orphan planets were announced. These seem to roam the cosmos with no star to pin them into an orbit. The Sigma Orionis star cluster appears as the bright star-like object near the bottom of this image. The cluster is a favorite hunting grounds of planet-searchers. The
image also shows the famous Horsehead Nebula and Orion's
belt, part of the Orion constellation. The belt's three stars are the brightest in the image, running from the upper right corner downward
and leftward. Below the leftmost of these three is the Horsehead Nebula. Follow the direction of the "horse's" neck
and you reach Sigma Orionis, to the right and slightly downward. (Courtesy Digitized Sky Survey, ESA/ESO/NASA FITS Liberator;
Color Composite: Davide De Martin (Skyfactory)) Send us a comment
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Now, four researchers claim that at least one of these may not be a free-floater after all. It might instead just be on a gravitational leash so long, it’s part of by far the vastest known planetary system, dozens of times wider than ours.
Not everyone is convinced.
Nonetheless, at a time when some scientists have begun exuberantly suggesting that the cosmos is buzzing with roaming planets, the four say it may be time to step back and reconsider what a planetary system is.
Perhaps “not all ‘isolated’ planetary-mass objects are really isolated,” they wrote in a paper to appear in the research journal
Astronomy & Astrophysics.
But they admit they’ve shown only that a candidate planet and its star are close enough to orbit each other, not that they do. That’s “pretty speculative,” wrote Phil Lucas of the University of Hertfordshire in Hatfield, U.K., co-discoverer of the first reported drifters, in an email.
The estimates in the new paper put the two bodies so far apart that their mutual gravitational pull would be a feeble 1/4,000 of that between the Sun and Earth. The authors, José Antonio Caballero of the Instituto de Astrofisica de Canarias in Tenerife, Spain and colleagues, wrote that the system seems unlikely to survive without being yanked apart by competing gravitational forces nearby.
But this is what makes the find intriguing, Caballero wrote in an email: it might represent a planet’s formative moments, “just before being ejected from the planetary system. Very probably many of the isolated planetary-mass objects that we find form in the same way.”
Researchers call the meandering worlds “isolated planetary mass objects” to account for the possibility that they differ fundamentally from familiar planets. More than 100 of them have been reported—which also challenges planet-formation theories, since planets are thought to form from dusty clouds surrounding their star.
Caballero’s team studied an object known as S Ori 68, listed as possibly belonging to the new breed. It inhabits a cluster of young stars called Sigma Orionis, 1,200 light-years away in the constellation Orion. A light-year is the distance light travels in a year.
S Ori 68, as massive as about five Jupiters, seems to be close enough to a brown dwarf star to orbit it, Caballero’s team wrote. A brown dwarf is a “failed” star that is too light to ignite nuclear fusion, the process that powers normal stars. This brown dwarf, called SE 70, turned up in recent sky surveys, they added; it seems to weigh as much as about 45 Jupiters and to lie around 1,700 times further from the planet as our Sun is from us.
This would make the system at least around 56 times wider than our Solar System as measured from the Sun to Neptune, now officially the furthest planet. The newfound system would also be around either seven or 20 times wider than the vastest such systems known. (The uncertainty is because one of those isn’t definitely known to be a planetary system.)
Caballero calls the planet-dwarf pair a “planetary system” rather than solar system because “solar” implies a normal star, like the Sun.
His team studied the brown dwarf using a spectrograph, a device to measure the spectrum of starlight, on the William Herschel Telescope on La Palma, Spain. It will take super-precise measurements of the two bodies’ motions to ascertain that they’re orbiting each other, Caballero and colleagues wrote, but this task may have to await the next generation of telescopes to become practical.
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Strange new objects that redefine the term “planet” are constantly coming to light. But even astronomers used to this got a jolt six years ago, with the announced discovery of apparent orphan planets. These seem to roam the cosmos aimlessly, with no star to pin them into a set orbit. Now, some astronomers claim that at least one of these objects may not be a free-floater after all. It might instead just be on a gravitational leash so long, it’s part of by far the vastest known planetary system, more than 50 times wider than ours. Not everyone is convinced. Nonetheless, at a time when some scientists have begun exuberantly suggesting that the cosmos is buzzing with unchained worlds, the researchers in the new study say it may be time to step back and reconsider our concept of a solar system. If the findings prove correct, “not all ‘isolated’ planetary-mass objects are really isolated,” wrote Jose Antonio Caballero of the Instituto de Astrofisica de Canarias in Tenerife, Spain and colleagues in a paper describing their findings. The work is to appear in the research journal Astronomy & Astrophysics. But they admit they have shown only that a candidate planet and its star are close enough to orbit each other, not that they do. It’s “pretty speculative,” wrote Phil Lucas, an astronomer at the University of Hertfordshire in Hatfield, U.K.—a co-discoverer of the first reported free-floaters—in an email. If Caballero’s estimates are accurate, the orbiting bodies would be so far apart that their gravitational attraction on each other would be markedly feeble, roughly 1/4,000 of that between the Sun and Earth. “It seems dubious that the system can survive” without being torn apart by competing gravitational forces nearby, they wrote. But this is what makes the find interesting, Caballero added in an email: his team might have captured “the very moment of the formation of an isolated planetary-mass object, just before being ejected from the planetary system. Very probably many of the isolated planetary-mass objects that we find form in the same way.” Caballero’s team examined an object known as S Ori 68, formerly listed as a possible free-roaming planet, in a young star cluster called sigma Orionis, 1,200 light-years from Earth in the constellation Orion. A light-year is the distance light travels in a year. Researchers announced the sighting of the apparent free-roamer in 2000, and over 100 have been reported since then. These findings also challenged planet-formation theories, because planets are thought to form from gas and dust surrounding their parent star. S Ori 68, which weighs about five times as much as Jupiter, seems to be close enough to a brown dwarf star to orbit it, Caballero’s team wrote. A brown dwarf is a “failed” star that is too light to ignite nuclear fusion, the process that powers normal stars. This brown dwarf, known as SE 70, came to light in stellar surveys in the past few years, Caballero wrote. He calls the supposedly orbiting bodies a “planetary system” rather than solar system because “solar” implies a normal star, like the Sun. His team estimates that the brown dwarf to weigh as much as 45 Jupiters, or 1/23 the weight of our Sun, and to be around 1,700 times farther from the planet as our Sun is from us. This would also make the new “planetary system” at least around 56 times wider than our Solar System as measured from the Sun out to Neptune, now officially the furthest planet. The newly described system would also be around either seven or 20 times wider than the vastest such systems known. (The uncertainty is because one of those isn’t definitely known to be a planetary system.) The astronomers studied the brown dwarf using a spectrograph on the William Herschel Telescope on La Palma, Spain. A spectrograph breaks up light into its constituent parts and records the resulting spectrum. Super-accurate measurements of the two bodies’ motions will be needed to ascertain that they are really orbiting each other, Caballero and colleagues wrote, but this task may have to await the next generation of telescopes to become practical.