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The fictional exomoon of Pandora from the hit movie 'Avatar' orbits its host gas giant planet.
Image: Kepler-16b is the first exoplanet disc
Exquisite Exoplanetary Art
Sept. 19, 2011 --
They're alien worlds orbiting distant stars far out of reach of detailed imaging by even our most advanced telescopes. And yet, day after day, we see vivid imaginings of these extrasolar planets with the help of the most talented space artists. The definition of an extrasolar planet -- or "exoplanet" -- is simply a planetary body orbiting a star beyond our solar system, and nearly 700 of these extrasolar worlds have been discovered so far (plus hundreds more "candidate" worlds). With the help of NASA's Kepler space telescope, the ESO's High Accuracy Radial velocity Planet Searcher (HARPS), French COROT space telescope and various other advanced exoplanet-hunting observatories, we are getting very good at detecting these worlds, but to glean some of the detail, we depend on artist's interpretations of fuzzy astronomical images and spectral analyses. That's the way it will be until we build a vast telescope that can directly image an exoplanet's atmosphere or physically travel to an alien star system. So, with the flurry of recent exoplanet discoveries, Discovery News has collected a few of the dazzling pieces of art born from one of the most profound searches mankind has ever carried out: the search for alien worlds orbiting other stars; a journey that may ultimately turn up a true "Earth-like" world.
Image: An exoplanet passes in front of (or "t
As an exoplanet passes in front of its star as viewed from Earth, a very slight dip in starlight brightness is detected. Observatories such as NASA's Kepler space telescope use this "transit method" to great effect, constantly detecting new worlds.
Some exoplanets orbit close to their parent stars. Due to their close proximity and generally large size, worlds known as "hot Jupiters" are easier to spot than their smaller, more distant-orbiting cousins.
Image: An artist's impression of Gliese 581d,
The primary thrust of exoplanet hunting is to find small, rocky worlds that orbit within their stars' "habitable zones." The habitable zone, also known as the "Goldilocks zone," is the region surrounding a star that is neither too hot nor too cold. At this sweet spot, liquid water may exist on the exoplanet's surface. Where there's water, there's the potential for life.
Credit: David A. Aguilar (CfA)
Usually, exoplanet hunters look for the slight dimming of a star or a star's "wobble" to detect the presence of an exoplanet. However, in the case of Kepler-19c, its presence has been detected by analyzing its gravitational pull on another exoplanet, Kepler-19b. Kepler-19c is therefore the Phantom Menace of the exoplanet world.
Image: A cool world some distance from its st
The habitable zone seems to be the pinnacle of extraterrestrial living. If you're an alien with similar needs to life on Earth, then you'll need liquid water. If your planet exists outside your star's habitable zone, well, you're in trouble. Either your world will be frozen like a block of ice, or boiling like a kettle. But say if your world had the ability to extend your star's habitable zone? There may be some atmospheric factors that might keep water in a comfy liquid state. Even better, if you like deserts, a dry world could even be oddly beneficial.
Image: A "hot Jupiter" and its two hypothetic
Planets with a global magnetic field, like Earth, have some dazzling interactions with the winds emanating from their stars. The high-energy particles bombard the planet's atmosphere after being channeled by the magnetism. A wonderful auroral lightshow ensues. But say if there's an exoplanet, with a magnetosphere, orbiting really close to its star? Well, stand back! The entire world would become engulfed in a dancing show, 100-1000 times brighter than anything we see on Earth.
Credit: Adrian Mann, <a href="http://www.bisb
"Candidate" exoplanets are often mentioned, especially when talking about detections by the Kepler space telescope. But what does this mean? As a world passes in front of its star, slightly dimming the starlight, this isn't considered a "confirmed" exoplanet detection. To make sure that signal is real, more orbital passes of the exoplanet need to be logged before a bona fide discovery can be announced. Until then, these preliminary detections are called exoplanet candidates.
Image: An exoplanet being destroyed by X-rays
Angry Suns, Naked Planets
Exoplanets come in all sizes and all states of chaos. Some might have wonky orbits, others might be getting naked. Other times, they're simply being ripped apart by X-rays blasted from their parent star. Bummer.
Image: Artist's impression shows HD 85512b, a
Super-Earths get a lot of press. Mainly because "Earth" is mentioned. Sadly, most of these worlds are likely completely different to anything we'd call "Earth." And you can forget calling the vast majority of them "Earth-like." It's simply a size thing -- they're bigger than Earth, yet a lot smaller than Jupiter, hence their name, "super-Earth." Easy.
Credit: Adrian Mann, <a href="http://www.bisb
For now, we have to make do with artist's renditions of exoplanets for us to visualize how they may look in their alien star systems. However, plans are afoot to send an unmanned probe to an interstellar destination. Although these plans may be several decades off, seeing close-up photographs of these truly alien worlds will be well worth the wait.
As the exciting quest to discover smaller and smaller exoplanets trundles on, over 1,000 of these alien worlds have been discovered to date. But as our instrumentation becomes more advanced and techniques more sophisticated, will we ever be able to spot the hypothetical moons in orbit around those exoplanets? Today, the answer appears to be “YES!” But like all science on the raggedy edge of discovery, more study is needed before a definitive discovery can be confirmed.
Frustratingly, the very nature of the technique used to discover this exomoon candidate prevents any further study of this interesting object.
Exomoons are the natural satellites that are thought to orbit planets orbiting other stars. It stands to reason that, although our rich and diverse solar system may not be a “typical” star system, other planets in other planetary systems should possess natural satellites like most planets in our solar system do.
Hunting exoplanets is hard, but how would astronomers go about hunting exomoons in orbit around those already-hard-to-find exoplanets? It sounds like a near-impossible task, but it’s a task that is slowly becoming more plausible.
Using an odd quirk of general relativity, astronomers are able to detect transient events when one star (or other celestial object like a massive planet) drifts in front of a more distant star. If the alignment is just right, the space-time bending gravitational field of the closest object (the “lens”) will focus the starlight of the more distant star from our perspective. This focusing effect causes a transient brightening known as a “microlens.”
To detect microlensing events, astronomers need to use survey telescopes that can monitor large swathes of sky. The lensing objects are usually undiscovered star systems that only give their presence away by their effect on amplifying starlight as they drift through the galaxy; events that occur randomly. But should the light from a microlens be detected and alerts sent out to collaborating telescopes quick enough, the “light-curve” of the event can be recorded and astronomers can deduce many physical characteristics of the lensing system.
Most recently, an exoplanet was discovered in the galactic bulge of the Milky Way (some 25,000 light-years away) orbiting within its star’s habitable zone — the first discovery of its kind. That object was spotted by three microlens surveys; the Microlensing Observations in Astrophysics (MOA — New Zealand/Japan), Optical Gravitational Lensing Experiment (OGLE — Poland) and Wise Observatory (Israel). Now, a detection by the MOA-II telescope at Mt. John University Observatory (MJUO) in New Zealand appears to have uncovered an exoplanet with an exomoon in orbit only 1,600 light-years from Earth in the direction of the Milky Way’s galactic bulge in the constellation Sagittarius. However, this particular exoplanet is a free-floating exoplanet with no host star.
MOA-2011-BLG-262 was a very short duration event detected on June 26, 2011 that appears to have been caused by a planet up to 4 times the mass of Jupiter and the companion moon is likely half the mass of Earth. The candidate exomoon is around 45 million kilometers (0.13 AU) from its host exoplanet. As a comparison, Jupiter’s most distant satellite (S/2003 J 2) orbits over 30 million kilometers from the gas giant, so such an extreme orbit around a larger planet is certainly feasible.
Sadly, as the amount of data collected from MOA-2011-BLG-262 was limited to that one event, there cannot be followup studies of the same system — although several telescopes were quickly alerted soon after MOA-2011-BLG-262 was detected.
The collaboration of researches, who published their results to the arXiv pre-print service, are being cautious about announcing the discovery of a small exomoon however. As the discovery would be unprecedented they have to consider other possibilities: Could, for example, the microlensing event actually have been caused by a star with exoplanet in tow (as opposed to a free-floating exioplanet and exomoon in tow), located much further away from Earth?
“Such a new discovery (of exoplanet and exomoon) would require strong evidence, so our favored model for this event is that it is a low-mass star or brown dwarf orbited by a planet of about Neptune’s mass,” the astronomers write.
But the exomoon evidence is tantalizing, a fact that will motivate intense scrutiny surveys that are becoming more and more sensitive to the presence of exomoons in microlensing light-curves.