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.
For the first time, astronomers have discovered a sun-like star playing host to a “habitable zone” exoplanet located inside the Milky Way’s galactic bulge — some 25,000 light-years distant — using a quirk of Einstein’s general relativity.
But don’t go having dreams of exotic getaways to the glistening lights of the center of our galaxy, this exoplanet is a huge gas giant world, about five times the mass of Jupiter. However, there is something (potentially) very exciting about this new discovery. Like Jupiter, this newly discovered giant exoplanet may possess small satellites; exomoons that could have life-giving potential.
This massive world was detected through “microlensing.” 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 in the brightening’s “lightcurve.”
Detailed in a pre-print published on the arXiv, the microlensing event, known as “MOA-2011-BLG-293Lb,” was detected by all three microlensing surveys: the Microlensing Observations in Astrophysics (MOA — New Zealand/Japan), Optical Gravitational Lensing Experiment (OGLE — Poland) and Wise Observatory (Israel). Astronomers then decided to do followup observations of the event using the adaptive optics instrumentation of Hawaii-based Keck Observatory. Keck’s adaptive optics are capable of removing atmospheric aberrations and returning precise measurements of this transient brightening.
Measurements of the MOA-2011-BLG-293Lb’s lightcurve returned information about the lensing star, its planet, their masses and the star-planet distance. The star, determined to be a G-type dwarf star slightly less massive than the sun, is located only 1.1 AU (astronomical units) from the planet. This makes it highly likely (a probability of 53 percent) that the massive exoplanet is orbiting within the star’s habitable zone, albeit on the outer edge, near the star’s “snowline.” This is the first such discovery to be made of a world located in the galactic bulge.
The habitable zone around any star is the orbital distance at which it’s not “too hot” and not “too cold” for liquid water to exist on a hypothetical rocky planet’s surface. It is for this reason that the habitable zone is also nicknamed the “Goldilocks Zone” — it’s just right for liquid water to persist. Liquid water is the stuff believed to be essential for the evolution of life as we know it.
The search for exoplanets of approximately Earth-sized dimensions has been feverish over recent years, but unless we can find planets orbiting within habitable zones of their stars with the potential of possessing liquid water, the life-giving potential of those small worlds seems slim.
But, as the multinational team of astronomers who analyzed MOA-2011-BLG-293Lb hypothesize, the super-Jupiter could play host to its own system of exomoons that would also exist on the outer edge of the star’s habitable zone. Alas, microlensing surveys don’t have the sensitivity (yet) to detect these diminutive satellites, but it is a tantalizing thought.
“Indeed, although the data do not explicitly show any signature of a companion to the Jupiter planet, this possibility is not ruled out,” the researchers write. “The planet is apparently at the edge between the snow line and the habitable zone, but considering a potential greenhouse warming effect, the surface temperature of a possible companion (exomoon) can be suitable for habitability.”
Like the recent discovery of MOA-2011-BLG-322, another microlensing event that detected the presence of another massive exoplanet in the galactic core, this discovery challenges some planetary formation theories. In this case, a super-Jupiter has been discovered orbiting its star at an equivalent distance the Earth orbits the sun. The MOA-2011-BLG-322 and MOA-2011-BLG-293Lb massive exoplanets orbit too close to their stars for them to have formed there. So some mechanism is either causing them to migrate from wider (beyond the systems’ snow lines) to more compact orbits (where they are observed today) or another planet-forming mechanism is at play.
Just the fact that we can detect the presence of exoplanets 25,000 light-years away inside the central bulge of our galaxy is mind-blowing enough, but the fact that this exoplanet exists so close to its star that its (hypothetical) satellites may be warm enough to possess liquid water on their surfaces, well, that really sets the imagination on fire as to the potential for life inside the crowded stellar party that is our galaxy’s central bulge.
Image: The view from a hypothetical exomoon orbiting a massive gas giant. Credit: ESO