Lost, Lonely Jupiter-Like World Without A Home
Silent and cold, the strange and eerie space between stars is a bewildering, bewitching, and mysterious place where orphaned planets wander without a stellar parent to call their own. Once, long ago, these free-floaters had a glowing parent-star, as well as planetary siblings, but they suffered a rude and terrible rejection as a result of gravitational perturbations that cast them out into the interstellar wilderness. Rejected, ejected children of an alien star beyond our Sun, these tragic planetary orphans were cruelly thrown out of their homes to fend for themselves in the frigid, hostile blackness, far, far away from the delightful warmth and sparkling brilliance of the stellar parents. In April 2016, a team of astronomers from the Carnegie Institution for Science in Washington D.C. and the University of Western Ontario in Canada, announced their discovery of one of the brightest and youngest of known free-floating planets. At the tender age of only 10 million years–a mere wink of the eye on galactic time scales–this planet-like object is between four and eight times the mass of Jupiter–placing it in the mass range between a giant planet and a fascinating stellar failure called a brown dwarf.
The planetary runaway identified as 2MASSJ1119-1137 was spotted by the astronomers using data derived from NASA’s Wide-field Infrared Survey Explorer (WISE), as well as ground-based telescopes. 2MASSJ1119-1137 was discovered as a result of its unique light signature by astronomers using a combination of optical and infrared images from large-area surveys of the sky.
“We identified 2MASSJ1119-1137 by its highly unusual light signature. It emits much more light in the infrared part of the spectrum than it would be expected to if it had already aged and cooled,” explained Kendra Kellogg in an April 6, 2016 Carnegie Institution For Science Press Release. Ms. Kellogg, the lead author of the paper describing this discovery, is a graduate student in the University of Western Ontario’s Department of Physics and Astronomy.
Oh, You Rogue!
Free-floating planets, sometimes referred to as rogue planets, interstellar planets, nomad planets, orphan planets, wandering planets or starless planets, are pitiful worlds without a home, and they orbit our Milky Way Galaxy directly–rather than circling a parent-star.
These unhappy planetary runaways are the victims of the violent turbulence that characterizes young solar systems, where highly disruptive interactions occur between neonatal protoplanets and parent stars. These violent interactions can ultimately result in the ejection of a doomed planetary newborn from the system that gave birth to it. Our Sun’s own familiar family might well have given birth to many more planets than the eight major planets that we see today, and these runaway siblings of our Earth may well be wandering around in the wilderness of the space between stars with no parent star to guide them. Astronomers had predicted the existence of such tragic planetary orphans for a long time before they were finally detected.
For over twenty years, exoplanets have been discovered at a rapid pace, with the number adding up to 2107 planets, dwelling in 1349 planetary systems, including 511 planetary systems as of April 2, 2016. Many of these very distant worlds, the planetary children of an alien sun, have been detected by indirect methods, such as a tattle tale wobbling or dimming of their parent stars caused by the influence of the planet.
Our barred-spiral Galaxy may be literally filled with Jupiter-sized free-floating exoplanets–and there could well be billions of them wandering starless around the center of our Milky Way–in a manner that is similar to how our Sun and other stars do. However, these planetary nomads are not stars, they are planets, and they have no star of their own. Once, long ago, they did have a parent-star like other planets, but that is no longer the sad case.
The absence of a searing-hot, fiery, glaring parent-star could actually wind up being helpful to observers, enabling them to gain a better understanding of the alien characteristics of these strange worlds.
Some objects of planetary-mass are thought to have been born in much the same way as stars, and the International Astronomical Union (IAU) has suggested that those objects be designated sub-brown-dwarfs.
All stars are born within especially dense blobs embedded within the whirling, ruffling billows of gigantic, cold, and dark molecular clouds that haunt our Galaxy in huge numbers. Like beautiful, beguiling phantoms swirling around in the wilderness of interstellar space, these strange and enormous clouds serve as cradles for new, young baby stars. Eventually, the dense blob collapses under the irresistible pull of its own gravity–and this is what lights a newborn star’s brilliant stellar fires. The fire will rage and burn with fabulous flames for as long as the new star “lives”, for that is how a star is born.
Astronomers have made great strides in detecting exoplanets orbiting stars beyond our Sun. Thousands have been discovered over the past two decades, and most of these brave new worlds were found hugging their parent-star, finishing one complete orbit (or year) in mere hours, days, or weeks. However, others have been spied orbiting as far out as Earth is to our Sun, taking one Earth-year to complete a circle around a shining parent-star.
But what about those very alien lost worlds that travel through our Galaxy alone, with no parent-star to guide them in their wanderings?
Most methods of spotting exoplanets depend on the periodicity of the distant world as it circles its parent-star and, therefore, cannot be used to detect lonely, starless, free-floating planetary nomads. However, there are two methods that have been devised to successfully spot these rogue worlds: gravitational microlensing and direct imaging.
Direct imaging enables curious astronomers to observe these lonely worlds continuously. Alas, only young and massive rogues can be spotted this way. This is because they are the only ones that send forth sufficient radiation to be discovered. Alternatively, in the absence of the blinding glare of the parent-star, an exoplanet itself can be seen much more easily–once it has been discovered.
When an object, that is the size of a planet, floats in front of a background star, its gravitational field results in a brief increase in the visible brightness of the background star. This is termed gravitational microlensing. Microlensing cannot be observed continually, but it doesn help astronomers detect older and lower-mass exoplanets than is possible through direct imaging.
Back in 1998, Dr. David J. Stevenson of the California Institute of Technology in Pasadena, proposed that there may be some planet-sized objects cast adrift in the vastness of the frigid space between stars–and that these lost worlds could possibly hold on to a thick atmosphere that would not freeze out. Dr. Stevenson went on to theorize that these atmospheres could be sustained by the pressure-induced far-infrared radiation opacity of a very thick hydrogen-bearing atmosphere.
Many astronomers propose that during the era of solar system formation, several small protoplanetary bodies may be thrown out of the forming system. With the reduced amount of ultraviolet light that would normally strip away the lighter ingredients from an atmosphere–because of its increasing distance from its stellar-parent–the rogue’s mostly hydrogen-and helium-containing atmosphere would be readily confined even by a relatively small Earth’s-sized object’s gravity. It has further been suggested that rogue planets with large liquid-water oceans may exist–and the presence of liquid water is important for the evolution of life as we know it. Also, these nomadic and lonely worlds are likely to stay geologically active for extensive periods of time, providing a geodynamo-created protective magnetosphere and potential sea floor volcanism that could conceivably provide an energy source for living tidbits to emerge on these starless, strange worlds. Therefore, Earthlings could theoretically, at least, live on a starless rogue planet wandering through the cold and vast expanse of interstellar space–although food sources would be quite limited. These planetary nomads would likely be difficult to spot as a result of their intrinsically weak thermal microwave radiation emissions flowing out from the lower portions of the atmosphere. However, some research suggests that reflected stellar radiation and far-infrared thermal emissions may be detected–that is, if one of these wandering worlds traveled within 1000 astronomical units (AU) of Earth. One AU is equal to the average distance between Earth and Sun, which is about 93,000,000 miles.
Another study of simulated planet ejection models indicates that possibly five percent of Earth-sized rejected and ejected exoplanets with Moon-sized moons of their own, could keep a grip on their natural satellites even after having been booted out of their parent-star’s family.
Lost, Lonely Jupiter-Like World Without A Home!
Free-floating planets are hard to distinguish from a myriad of potential interlopers. “Much more commonly, distant old and red stars residing in the far corners of our Galaxy can display the same characteristics as nearby planet-like objects. When the light from the distant stars passes through the large expanses of dust in our Galaxy on its way to our telescopes, the light gets reddened so these stars can pose as potentially exciting nearby young planet-like objects in our data, when they actually are not that at all,” explained Dr. Jacqueline Faherty in the April 6, 2016 Carnegie Press Release. Dr. Faherty is a Hubble Fellow at Carnegie.
With an understanding that these frequent misidentifications occur, the team of astronomers promptly checked their findings using the FLAMINGOS-2 spectrograph instrument on the Gemini South Telescope in Chile.
“We promptly confirmed that 2MASS J1119-1137 is in fact a young low-mass object in the solar neighborhood, and not a distant reddened star,” commented Dr. Stanimir Metchev in the same Press Release. Dr. Metchev is of the University of Western Ontario.
Next, the astronomers sought to determine the exact age of this strange, wandering starless object. “Our Gemini observations only showed that the object was younger than about 200 million years. If it was much younger, it could actually be a free- floating planet–an analog of our own Jupiter, yet without a host star,” Dr. Metchev added.
The last clue to this intriguing mystery was contributed by Dr. Jonathan Gagne of the Carnegie Institution. Dr. Gagne used one of the most efficient instruments for infrared spectrometry in existence, the FIRE spectrograph on Carnegie’s Baade 6.5-m Telescope in Chile. FIRE enabled him to measure the line-of-sight velocity of 2MASSJ1119-1137 through the Doppler shift of its emitted light. By combining this measurement with the motions in the sky of 2MASSJ1119-1137, the team of astronomers discovered that it belongs to the most youthful collection of stars in our own Solar System’s neighborhood. This group of stars harbors about two dozen relatively young 10 million-year-old stars, that are all traveling together through interstellar space. This stellar group is known as the TW Hydrae association.
“Demonstrating that 2MASSJ1119-1137 belongs in the TW Hydrae association, and so is only 10 million years old, inevitably led to the exciting conclusion,” Dr. Gagne continued to explain.
Because the nearby 2MASSJ1119-1137 is only about 95 light-years from Earth, it just misses the title for being the brightest free-floating planet analog. That designation is held by a different object dubbed PSOJ318.5?22, which was discovered in 2013. However, at an age of 23 million years, PSOJ318.5?22 is more than double the age of 2MASSJ1119-1137, and is therefore more massive.
Dr. Kellogg noted in the April 6, 2016 Carnegie Press Release that “Discovering free-floating planet analogs like 2MASSJ1119-1137 and PSOJ318.5?22 offers a great opportunity to study the nature of giant planets outside the Solar System.” He added that free-floating planet candidates are “much easier to scrutinize than planets orbiting around other stars. Objects like 2MASSJ1119-1137 are drifting in space all alone and our observations are not overwhelmed by the brightness of a host star next door.”
The paper describing this research will be published in The Astrophysical Journal Letters.
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