Hunting for the Real 'Planet X'
Is there a large object at the edge of the solar system causing a sudden drop-off in Kuiper Belt population?
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 story of Planet X describes a fascinating journey of astronomical history and scientific discovery.
The announcement of the discovery of exoplanet Alpha Centauri Bb on Oct. 16 is a testimony to how far planetary detection techniques have come over the last few decades.
It brings the total of confirmed exoplanets -- or "extra-solar planets" -- to a staggering 825. However, the search for planets in our own solar system has subsided since the pioneering days at the end of the 18th century with the discovery of Uranus and almost one hundred years later with the identification of Neptune. The idea of another planet, dubbed 'Planet X,' inspired astronomers to keep searching for yet another 100 years in a hunt that was full of twists and turns.
The hunt for Planet X began in 1781 when British astronomer Sir William Herschel was studying stars in the constellation of Taurus and noticed one star seemed slightly fuzzy or nebulous in appearance. A few days later it seemed to have moved position -- he concluded it was a comet. Further study revealed it was actually a planet -- Uranus -- the seventh planet in our solar system and beyond the orbit of Saturn.
Detailed observations of Uranus' movement revealed an orbit that seemed to be influenced by another, even more distant, object. Mathematicians studying the data predicted the position of an eighth planet before it was officially discovered. Visual confirmation of Neptune's existence was announced in 1846.
Using the same techniques to study the orbital characteristics of both Uranus and Neptune revealed they were both still being tugged at by the gravitational force from another unknown object. The search for the ninth planet in the solar system began and it was American astronomer Percival Lowell who identified possible candidates.
Some years after Lowell's death in 1930, Pluto was identified by Clyde Tombaugh (an astronomer working at Lowell Observatory) and was believed to be the final member of the solar system's planetary family.
Is there a large object at the edge of the solar system causing a sudden drop-off in Kuiper Belt population? NASA/ESA
However, the 1978 discovery of Pluto's moon Charon reopened the Planet X debate. Through accurate measurements of Charon's orbit, the mass of Pluto could be deduced. Ultimately it showed that the 'ninth planet' couldn't possibly have affected the orbits of Uranus and Neptune as observations appeared to show.
The renewed interest in Planet X was short lived as the Neptunian flyby by Voyager 2 in 1989 revealed its mass was less than thought. By reapplying this knowledge showed the outermost "ice giant" planets were behaving exactly as they should and the orbital perturbations were doewn to observational error. It seems the myth of Planet X had finally died.
This could have been the end of the Planet X saga, but recent studies of the Kuiper Belt -- a region of icy minor planets located in the outermost reaches of the solar system -- suggest this may not be the case.
It would be reasonable to expect the millions of frozen lumps of rock would gradually decrease with distance from the sun, but at a distance of 48 Astronomical Units (beyond the orbit of Pluto) they seem to drop off suddenly, at the so called "Kuiper Cliff."
Maybe Planet X is responsible for this strange unexpected feature in the outer edge of our solar system… or maybe not.
The Voyager and Pioneer spacecraft heading out of our solar system haven't detected any substantial planets that might cause the 'cliff,' but space is vast; the chances of a spacecraft happening to fly past a previously undiscovered world would be highly unlikely. Also, ground-based observatories and space telescopes (like NASA's Wide-Field Infrared Survey Explorer) have turned up little evidence.
But the jury is still out. Until an answer is found for the Kuiper Cliff, the ghost of Planet X will remain as a tantalizing, but unlikely, explanation.