Kepler Watches White Dwarf Warp Spacetime
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 Kepler space telescope’s prime objective is to hunt for small worlds orbiting distant stars, but that doesn’t mean it’s not going to detect some extreme relativistic phenomena along the way.
While monitoring a red dwarf star — designated KOI-256 — astronomers detected a dip in starlight in the Kepler data. The NASA space telescope is constantly on the lookout for these dips as they can be an indicator of an extrasolar planet passing in front of the star’s disk. This event is known as a “transit” and Kepler has the unprecedented sensitivity to detect sub-Earth-sized worlds drift in front of their host stars.
When a transit was detected in the KOI-256 system, researchers led by Caltech’s Phil Muirhead thought they’d just witnessed a massive planet orbiting the star. However, something was very strange about this particular transit.
“We saw what appeared to be huge dips in the light from the star, and suspected it was from a giant planet, roughly the size of Jupiter, passing in front,” said Muirhead.
Using the ground-based Palomar Observatory in California, Muirhead’s team applied another exoplanet-hunting technique to KOI-256. The “radial velocity method” can detect worlds in orbit around other stars through the careful analysis of the spectrum of starlight. If an exoplanet is in orbit, the mass of the world will gravitationally “tug” on the host star. This tugging creates a slight wobble, generating a red- and blue-shifting of light; a tell-tail sign that a planet is there.
Radial velocity measurements of KOI-256, however, revealed that something else was there — and it certainly wasn’t an exoplanet. The star was found to be wobbling “like a spinning top” meaning something way more massive is in orbit — a compact white dwarf star, the stellar husk of a burned-out star.
Although red dwarfs are small, white dwarfs are even smaller, but very, very dense. The white dwarf in the KOI-256 binary is about the size of Earth and yet packs the mass of the sun. “It’s so hefty that the red dwarf, though larger in physical size, is circling around the white dwarf,” added Muirhead.
Most of the stars in our galaxy are binary stars; two stars in a tight cosmic dance is not a rarity.
With the help of another NASA space observatory, the Galaxy Evolution Explorer (GALEX), which analyzes the ultraviolet light of the stars in Kepler’s field of view, the researchers noticed that as the white dwarf passed behind the red dwarf, the starlight would dim, but when the white dwarf passed in front, the light would be slightly brighter than expected. This is counter-intuitive to how transits work, but KOI-256 is anything but intuitive.
As the white dwarf passed in front of the red dwarf, its extreme gravitational field was causing spacetime to bend, focusing the light from the red dwarf, enhancing the starlight. As the white dwarf passed behind the red dwarf, there would be no gravitational disruption of starlight and therefore no starlight enhancement. This finding will be published on April 20 in the Astrophysical Journal.
“Only Kepler could detect this tiny, tiny effect,” said Doug Hudgins, Kepler program scientist at NASA Headquarters, Washington. “But with this detection, we are witnessing Einstein’s theory of general relativity at play in a far-flung star system.”
Indeed, this relativistic effect, known as “microlensing,” has been used to detect exoplanets before, but this is one of the first examples of a binary stellar partner being detected through the analysis of the light of its lower-mass sibling.
Microlensing is a transient event, but large-scale lensing events have been recorded in deep space. For example, the Hubble Space Telescope has identified arcs of light surrounding massive galactic clusters as distant light being bent around warped spacetime. This light is often from distant galaxies behind the galactic clusters, magnifying and focusing the ancient galaxy’s light.
These are nature’s natural cosmic magnifying lenses, and with this new white dwarf discovery, Einstein has teamed up with Kepler to reveal a fascinating twist in our hunt for extrasolar planets (and, now, white dwarfs).
Image: Artist’s impression of the white dwarf warping spacetime and bending the light of the red dwarf. Credit: NASA/JPL-Caltech