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.
Astronomers have discovered a giant world orbiting a star using a quirk of Einstein’s general relativity. This “microlensing” event enabled astronomers to test a new survey technique to detect the alien world from over 25,000 light-years away, deep inside the Milky Way’s galactic bulge.
Microlensing events occur when a star passes in front of another more distant star. As the nearer star passes in front, its gravitational field — which is (according to general relativity) bending the surrounding spacetime — deflects the light from the more distant star. Like the lens in a magnifying glass, the starlight is magnified and Earth-bound observatories are able to spot a transient brightening. Information about the “lens” (the foreground star) and any planets in tow can then be deduced.
The microlensing event “MOA-2011-BLG-322″ was detected during a 2011 observing season of a collaboration of observatories. Astronomers of the Microlensing Observations in Astrophysics (MOA — New Zealand/Japan), Optical Gravitational Lensing Experiment (OGLE — Poland) and Wise (Israel) all reported the event. Of the 218 microlensing events detected during that season, only 80 were confirmed by all three networks. Of those 80, three showed signs of a clear “planetary anomaly.”
A planetary anomaly is caused by a secondary mass (i.e. a planet) creating its own spacetime warping, adding detail to the microlensed starlight. Microlensing has therefore become a useful tool in the search for exoplanets orbiting distant stars. This technique contrasts greatly with the two leading techniques of exoplanet detection — the “transit” technique (the dimming of starlight caused by an accompanying exoplanet passing in front) and the “radial velocity” technique (the wobble of a star caused by the gravitational tugging of an orbiting exoplanet).
Although exoplanets have been discovered via microlensing before, this is the first planetary detection that uses only high cadence survey data after the event. Usually, when a microlensing event is detected, alerts are sent out to collaborating astronomers who then slew their telescopes toward the event and measurements are taken. In the case of MOA-2011-BLG-322, only data from the three surveys were used to estimate the star’s mass and the nature of the planetary companion. According to the researchers, this “shows that the survey data alone can be sufficient to characterize a planetary model.”
The exoplanet detected in this case has a mass of approximately eight times that of Jupiter and its star is likely an M-type star, around one third the mass of our sun. The exoplanet has an orbital distance of nearly 4 astronomical units (AU); or four-times the Earth-sun distance. From this observation, some interesting science can be done.
It would appear that MOA-2011-BLG-322′s massive planet exists in an orbit beyond its host star’s “snowline”. This is a region around any given star where protoplanetary material in the protoplanetary disk of a young star begins to freeze, making it a ripe environment for planets to form. However, this world appears to be too massive for its comparatively close orbit.
“According to the core accretion scenario, Jovian planets form beyond the snowlines of their parent stars, but massive planets around M-type stars should be rare, since their formation times are longer than the typical disk lifetime,” the researchers write. “In the disk instability planet-formation scenario massive planets do form around M stars, but at distances of over 7 AU.”
For such a large planet to be orbiting only 4 AU from its parent star, there appears to be some conflicts with existing planetary formation theories. Fortunately, microlensing is highly sensitive to detecting worlds beyond the snowlines of their stars, so with further discoveries by microlensing surveys will come better knowledge of how massive planets form or migrate from wider orbits.
“Unlike most of the microlensing-detected planets to date, the planet presented here was not detected in real time, but in a post-season analysis, illustrating the essence and elegance of the second-generation survey principle,” the researchers conclude.
This study has been submitted for publication in the Monthly Notices of the Royal Astronomical Society.
Image: A possible visualization of a massive exoplanet inside the Milky Way’s galactic bulge. Credit: ESO/L Calçada, NASA, edit by Ian O’Neill/Discovery News