Could Giant Exoplanets Support Habitable Exomoons?
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.
The 2009 blockbuster film “Avatar” presented a fairytale view of a paradise-like lush green moon called Pandora, orbiting a monster Jupiter-like planet.
As far as I can tell, the idea of habitable moons was first introduced into modern cinema in the 1977 sci-fi classic “Star Wars.” The forest moon of Endor — hideout for the Rebel Alliance — was conveniently eclipsed by a bloated Jovian planet when the Death Star arrived to blow the moon apart. That’s pretty lousy navigation for the Evil Empire’s pilots.
I thought this idea was a novelty until reality caught up with science fiction and we discovered planets orbiting other stars. In particular, astronomers found gas giant worlds nestled within the habitable zone of their stars, where temperatures are moderate enough for water to remain in liquid form. Continents would not be expected to be found on these giants. But, possibly, any moons orbiting them might be large enough to hold onto atmospheres and nurture life.
The satellite systems of Jupiter and Saturn are two distinct examples of what should be common throughout the galaxy.
At a frigid distance of 500 million miles from the sun, Jupiter has four major moons that are lifeless on the surface but geologically quite diverse. Two of them, Europa and Ganymede, would defrost into water worlds if brought to Earth’s distance from the sun.
The bulk of the mass in Saturn’s satellite system is tied up in Titan, a chilly world that is bigger than the planet Mercury. Surface temperatures of minus 300 degrees aside, the world is amazingly Earth-like with lakes, streams, a thick atmosphere, and volcanism.
The Hunt for Exomoons with Kepler (HEK) project at Harvard University uses Kepler data from over 3,000 exoplanet candidates to look for indirect evidence of moons orbiting giant planets. Any unseen moons would gravitationally tug on a planet and this would slightly alter the time of when the planet is measured passing in front of its star (transiting). The planet’s “mass transit” (pun intended) schedule would be off slightly. What’s more, if a large moon passes in front of or behind a planet it would cause unusual variations in its brightness of the star.
In a recent paper, Rene Heller of the Leibniz-Institute for Astrophysics Potsdam, Germany and Rory Barnes of the University of Washington in Seattle, predict that two exoplanets are good candidates for possessing habitable exomoons.
One, cataloged KOI211.01, is about one-third Jupiter’s mass and is in an Earth-sized orbit about a sun-like star. The other is Kepler 22b; located 600 light-years away, this candidate can be dubbed either a mini-Uranus or super-Earth, weighing in at about six Earth masses. The planet could be a big ball of water surrounding a rocky core. The HEK project has not detected any moons above about half an Earth mass orbiting this planet. (Titan is only 1/100th of Earth’s mass)
Exomoons are likely to be tidally locked to their planet just as our moon keeps one hemisphere facing Earth. Days would last for half of an orbital period. Seasons would be possible depending on the moon’s axial tilt plus that of the host planet, and how elliptical the planet’s orbit is about the central star. Eclipses of the parent star would happen frequently and this would alter the weather abruptly on a moon. Life might have a biorhythm in synch with the eclipses, just as some organisms on Earth sense the lunar phase cycle.
Living alongside a gas giant planet is likely rough on moons and could inhibit the emergence of life, say the researchers. If the moon is too close to the planet, tidal forces can make it so hot that the surface is always erupting volcanically. Jupiter’s innermost major satellite Io is an example of this.
A water-laden moon with a thick atmosphere might suffer from a runaway greenhouse effect. The water vapor, combined with other greenhouse gasses such as methane and carbon dioxide might make the moon too warm, as is the case of Venus. The closer the moon is to its gas giant planet, the more radiation it will absorb, simply from starlight reflected off the planet.
But if the moons are farther out they are not protected from galactic cosmic rays by the planet’s magnetic field, and are too small to generate a substantial field of their own. This radiation might sterilize the surface of life.
By analogy with the circumstellar habitable zone, these constraints define a circum-planetary “habitable edge,” say the researchers. They conclude that if either exoplanet hosted a moon at a distance greater than 10 planetary radii, then it could be habitable.
Characterizing such moons is certainly an order of magnitude or more difficult that identifying inhabited planets. So although there might easily be more inhabited moons in out galaxy than planets, characterizing them is a daunting task for far future super-telescopes… or maybe starships.
Image credit: NASA