'Dead' Stars May Host Living Worlds
David A. Aguilar (CfA)
A planetary nebula surrounds a white dwarf star with a hypothetical habitable planet in orbit.
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.
Earth-sized planets that host life should be far easier to find around parent stars that are white dwarfs, the ultimate incarnations of stars like the sun, a new study shows.
White dwarfs are the dense stellar cores that remain after a sun-like star runs out of fuel and goes through its expanding, red giant phase, a process that will consume its inner planets. In our solar system, for example, Mercury, Venus and possibly Earth will be destroyed when the sun evolves into a red giant some 4.5 billion years from now.
But the system won't necessarily be doomed.
Outer planets may migrate inward, closer to the star, and new worlds may form. Not all will be in stable orbits, but an Earth-sized world located about 1 million miles away from a host white dwarf star would have a temperature roughly the same as Earth’s. At that distance, the planet could have liquid water on its surface, a condition believed to be necessary for life.
Scientists are developing techniques to scan the atmospheres of planets beyond the solar system for oxygen and other chemical signs of life. It's a laborious and time-consuming process to separating out light passing through a planet’s atmosphere from all the background starlight.
But Earth-sized planets circling white dwarf stars, which are themselves about as big as Earth, make for much bigger needles in extrasolar planet haystacks.
Avi Loeb, a theorist at the Harvard-Smithsonian Center for Astrophysics, figures the upcoming James Webb Space Telescope, a successor to the Hubble observatory, would need only about five hours of observing time to look for biomarkers in the atmosphere of a planet circling in a white dwarf’s habitable zone.
“Usually the background star is so much brighter, it’s so much bigger than the planet that absorption (of light) due to the atmosphere is a very small signal that you have to fish out of the much more prominent emission from the background star,” Loeb told Discovery News.
“In the case of the white dwarf, it’s sort of the best of all circumstances, where the object that is blocking the star is of the same size as the star itself. That offers the best prospect for detecting the absorption due to the atmosphere, relative to the background light,” he said.
A planetary nebula surrounds a white dwarf star with a hypothetical habitable planet in orbit.David A. Aguilar (CfA)
Like NASA’s planet-hunting Kepler Space Telescope, the technique would only work for white dwarf systems that are aligned relative to the observatory’s line of sight so that orbiting planets pass in front of, or transit, their parent stars.
While the star is eclipsed, some light will pass through the planet’s atmosphere -- if it has one -- and leave telltale chemical fingerprints that can be detected by instruments in a telescope.
“If we happen to be situated so that we can see an eclipse, then the planet would block a substantial fraction of the light from the white dwarf. Then we can basically use the light that is passing through the atmosphere to figure out what the atmosphere is made of,” Loeb said.
Of key interest would be detections of oxygen, which on Earth is a clear sign of life from photosynthesis by plants.
“The only reason we have oxygen in the atmosphere right now is because of life,” Loeb said. “If you remove life from Earth, then within 1 million years or so, the oxygen will be completely depleted. It will make all kinds of molecules of oxidized metals, for example, and it will be consumed from the atmosphere.”
Scans of exoplanets’ atmospheres also could find water vapor and other potential biomarkers.
While there is not yet any direct evidence of planets circling white dwarfs, astronomers believe they exist. Previous studies have shown that as many as 30 percent of white dwarf stars have heavy elements on their surfaces, presumably from rocky bodies that broke up relatively soon after the white dwarf formed.
A planet could find a stable orbit in white dwarfs’ habitable zone, one that would have it circle its parent star in just 10 hours.
“I’m not saying that we definitely know that such planets are there, but it’s quite plausible that the system after a while cleans itself up and for over a billion years or more, it may have stable Earth-mass planets,” Loeb said.
“It sounds like a reasonable extrapolation for what we’ve seen,” added astronomer Marc Kuchner, with the Exoplanets and Stellar Astrophysics Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Md.
“There’s no reason to think you wouldn’t find one now and then," Kuchner told Discovery News.
The research will be published in an upcoming edition of the Monthly Notices of the Royal Astronomical Society and is available online via the arXiv preprint service.