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Science Matters: young exoplanet found
A rogue exoplanet recently found in the Milky Way by scientists could harbour alien life forms.
Once upon a time they seemed to belong to the make-believe world of science fiction. Then, like all good literature, they one day became science fact.
Defying the astro-sceptics, rogue exoplanets – planets outside our solar system that have been ejected by their host stars and thereafter roam through interstellar space – have been showing their enigmatic faces from time to time.
Now scientists have found the youngest and lightest rogue that formed a mere 12 million years ago – a relative infant by solar standards.
Untethered to an individual star by gravity, this wandering gaseous world – reported in the latest issue of the respected Astrophysical Journal Letters – lies an estimated 80 light-years away and roams footloose and fancy free through our galaxy, the Milky Way.
PSO J318.5-22, as the lone ranger is prosaically known, weighs in at more than six times that of our own gas giant, Jupiter, with a mass almost twice that of the solar system's other planets combined.
Some pundits say this solitary orb might even harbour alien life forms, albeit not necessarily as we would know them.
"If it hosts life, it would probably not exist in its own atmosphere, but rather in any large moons orbiting it, perhaps kept warm – and hence with liquid water – by tidal forces," says Melbourne University astrophysicist Alan Duffy. "It might be similar to Jupiter's moon Europa or perhaps Saturn's moon Enceladus."
So far, Dr Duffy says, astrophysicists have not found any orbiting moons. "But, as few objects ever form alone in the universe, I'd bet there must be a smaller body travelling with it."
This newly discovered, potential rogue displays the signatures that would be expected of a big exoplanet, rather than that of a small star, says Swinburne University astronomer Chris Fluke. "A rare discovery like this takes a very special combination of knowledge, insight, the right technology – and just a little bit of luck," Associate Professor Fluke says.
The Pan-STARRS 1 survey telescope on Hawaii's Haleakala volcano identified the cosmic orphan by its faint and unique heat signature. "The heat is probably from radioactive decay, as well as remaining heat from its formation," Dr Duffy says.
Other telescopes confirmed that the oddball orb has characteristics not dissimilar to gas giants orbiting young stars. The difference is that this one has no parent star.
"It's similar in mass to the so-called hot Jupiters that orbit close to their parent star – and yet, in this case, there is nothing nearby. It's all on its own," Dr Duffy says. "This is unheard of and means our ideas of how small an individual forming object can be need to be revised."
Not having a parent star means telescopes can scrutinise it without being disturbed by the host star's light. "So we can get exquisite spectra, or chemical fingerprints, of this object – perhaps even getting a direct image of it," Dr Duffy says.
The young rogue, he believes, is almost certainly a member of an association of young stars all travelling in the same direction, having formed together 10 million to 30 million years ago. "Because this new failed star is young, it hasn't had much time to drift away from a companion," he says.
Here lies a mystery: nothing in the known universe is believed to form in isolation. Therefore, Dr Duffy surmises, it's likely to be orbited by smaller planetesimals – objects bigger than boulders formed from dust, rock and other materials that sometimes evolve into planets with deep crusts of solid ice.
"So perhaps this lonely world isn't so lonely after all," he suggests.
The long search
Searching for PSO J318.5-22 was like trying to find a very red – so perhaps rusty – needle in a haystack, Dr Duffy says. "It's not very bright and so a lot of sky had to be searched to find this first example."
Some new telescopes are coming online, allowing astrophysicists to scan the entire sky in their search for even fainter objects. These new tools include the Large Synoptic Survey, which will search for small rocky rogues; and another, known by the acronym WFIRST, a former spy satellite given to NASA by the US National Reconnaissance Office, which will hunt for new gas-giant rogues.
"With luck, we might put this strange object in context as we fill in the gaps in the fascinating crossover between tiny stars and giant gas planets," Dr Duffy says.
The existence of rogues has been investigated theoretically for a long time, says CSIRO astrophysicist Kurt Liffman. "They were only first really detected a few years ago, using a technique called gravitational lensing." This method picks up the tiny, but just discernible, amplification of a star's brightness as an exoplanet passes between the star and telescope.
Theoreticians have deduced that rogue exoplanets may have a low-intensity infrared signature, he says. "In other words, they are typically much colder and fainter than newly formed stars."
So far, astronomers have catalogued more than 1000 exoplanets using indirect methods, including planet-induced wobbling or dimming of their parent stars.
Stars versus planets
Within our solar system, there are some strong similarities between the sun and Jupiter. Both comprise almost 75 per cent hydrogen and 25 per cent helium, with neither object having a solid surface.
The big difference, Associate Professor Fluke says, is the sun is a star, which means it generates light and heat through nuclear reactions in its core; Jupiter is neither hot enough nor heavy enough for this to occur.
"We know there are many other objects in our galaxy with masses somewhere between those of the sun and Jupiter," he says. "These are often referred to as brown dwarf stars, but there is no easy way to provide a clear distinction between a really big gas planet and a very small star."
Recent direct observations of exoplanets orbiting nearby stars have provided scientists with a much better idea of the "signature" of big planets.
"This includes properties like the amount of heat they give off," Associate Professor Fluke says. "Like using a pair of night goggles to observe a human's body heat in the dark, astronomers are able to detect the very faint glow of these objects with infrared telescopes."
Dime a dozen?
Astronomers believe the cosmos might teem with rogues. Some scientists believe they might be twice as common as stars in the observable universe.
More discoveries support this view. An exoplanet, labelled Fomalhaut b, was recently found to have an orbit so strange that astronomers resolved it was a rogue orb. Located roughly 25 light years away in the constellation of Piscis Austrinus, the strange world was revealed by the Hubble Space Telescope.
In another study, a two-year scan of the sky, an international team stumbled upon 10 rogue exoplanets drifting aimlessly between 10,000 and 20,000 light years away in the direction of our Milky Way's central galactic bulge.
As yet, no host stars have been detected within about 10 astronomical units of the rogues (or 10 times the distance between the sun and the Earth).
Such discoveries have not surprised some scientists. "I've long believed that there is more planetary real estate wandering the interstellar spaces than tethered to stars," says acclaimed physicist Paul Davies, director of the Beyond Centre at Arizona State University in the US.
Exactly how exoplanets become orphaned is open to debate. In his book The Eerie Silence (Allen Lane), Professor Davies writes: "Theoretical analysis of planetary motion suggests that orbits can be destabilised by planets 'ganging up', resulting in objects being flung out of a star system altogether.
"As a result, there could be many 'rogue planets' wandering the dark interstellar spaces, perhaps accompanied by a retinue of moons. Quite possibly our solar system started out with more than the eight planets we see today, the rest being ejected."
Rogue exoplanets might offer a marginally safer environment for life to thrive, away from the dangers of a solar system, Associate Professor Fluke suggests. "Without a sun to call their own, though, an alternative heat source is likely to be crucial."
Professor Davies' preferred scenario for life on these weird worlds is in the deep subsurface. "Many of them will be icy planetesimals, with cores that remain liquid for billions of years, offering possible abodes for microbial life."
Yet he struggles to comprehend how advanced life would get going. "Even if more complex biological entities were to evolve, one can only speculate what life would be like in such a location," Professor Davies writes. "How long would it take sentient beings, confined to their pitch-black liquid habitat by a solid sky hundreds of kilometres thick, to discover that there was a vast universe beyond their world's apparently impenetrable roof? It is hard to imagine that they would ever break out of their ice prison and beam radio messages across space."
The prospect of finding celestial rogues was first raised in 1999 in an article published in the British journal Nature. American astronomer David Stevenson wrote: "During planet formation, rock and ice embryos of the order of Earth's mass may be formed, some of which may be ejected from the solar system as they scatter gravitationally from proto-giant planets. These bodies can retain atmospheres rich in molecular hydrogen."
Bodies of this sort, he believes, might have oceans of water, "whose surface pressure and temperature are like those found at the base of Earth's oceans".
It seems like he was on to something.
Check out how the Pan-STARRS 1 survey telescope works at: http://pan-starrs.ifa.hawaii.edu/public/
Learn more about rogues at: www.eso.org/public/news/eso1245/
Discover how the Institute of Astronomy at the University of Hawaii first reported the new find at: www.ifa.hawaii.edu/info/press-releases/LonelyPlanet/
VCE Astrophysics – Detailed study 3.2 for Units 1 and/or 2 Physics – accessed at VCE Physics above, p21.
F-10 Physics (sub-strand "physical sciences" in AusVELS): ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
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