Super-Dense Planets May Be Boiled Gas Giant Dregs

Bizarre, super-dense chthonian planets may be the cores of evaporated gas giants. ->

Ever since the early 1990s, when exoplanets were first confirmed to exist, we seem to just keep finding more and more for which we have no existing definition.

The latest addition to the planetary menagerie? Chthonian planets. Super-dense planets, barely larger than Earth, but denser than iron. Now that they've been found, astronomers are trying to figure out how they form.

It's quite likely that these planets, discovered in data acquired by NASA's Kepler space telescope, are actually the exposed cores of former gas giants. Neptune-sized worlds which have been boiled away, leaving a tightly compressed core to drift naked in space.

PHOTOS: Top Exoplanets for Alien Life

At least, that's the most likely explanation for what these oddball worlds might be. None of the conventional planet formation theories can explain them!

Exposed Planetary Cores

Olivier Grasset, a geophysicist from the University of Nantes, France, is among those investigating what mechanisms could form such unusual planets. The conclusion which he and his colleagues reached was that these could be the "fossil cores" of Neptune-sized worlds.

After forming in the icy outer parts of their star systems, they would have then migrated inwards. Becoming hot neptunes, their volatile outer layers would then boil away leaving behind a compacted ball of rock and metal, like planetary dregs.

The interesting thing is that, while the planets recently discovered by Kepler are certainly new discoveries, they are by no means the first of this kind of planet to be found. The story actually begins a little over a decade ago...

ANALYSIS: Are Small Rocky Worlds Actually Naked Gas Giants?

In the late 1990s, another shocking planet was discovered. HD 209458b, nicknamed "Osiris" by its discoverers, is an extreme world, which took us all by surprise. It is 220 times as massive as Earth and orbiting so close to its parent star that its year lasted just three and a half Earth days. It turns out Osiris is actually evaporating away, leaving a vapor trail in its wake.

The conclusion drawn by many exoplanet researchers was that any planet which strayed too near to a star would probably evaporate away. Osiris itself is estimated to have already lost around 7 percent of its original mass. This led to a group of researchers, headed by Guillaume Hébrard to hypothesize a previously unknown type of planet. Given the name of chthonian planets, these were expected to be the extremely dense remnants of gas giants which had boiled away in the heat.

The first chthonian planet discovery was actually 10 years ago. CoRoT-7b, 490 light-years away, and with an orbit so close that one year on this planet would last just 20 hours. Its orbital radius of 0.017 AU puts it just four hundredths as far from its star as Mercury is from the sun. It also has somewhere between 5 and 11 times the mass of Earth packed into just 1.7 times Earth's radius.

While not much has been said recently about this bizarre flavor of planet, the new Kepler data shows that it's certainly not alone. With several such planets out there, they're now officially in a brand new class of their own.

Bake Me A Planet!

Chthonian planets didn't originally form the way Earth did. Instead, they formed in the crushing interior of a gas giant. Exactly what lies inside a planet like Jupiter is an open question, but pressures will easily be able to reach five million times the pressure at sea level on Earth – 500 gigapascals.

PHOTOS: Exquisite Exoplanetary Art

Temperatures are also expected to be extreme, reaching around 6000 Kelvin, which is about the same as the temperature on the surface of the sun. Worlds like these will certainly be no strangers to heat.

Under such extreme conditions, these planets will form to be extremely dense. Together with fellow researchers Antoine Mocquet, a planetary scientist also at Nantes, and Christophe Sotin, a planetary geologist working at NASA JPL, Olivier Grasset used a computer simulation to try and work out how a chthonian planet might form.

They found that If a Neptune-like world were to evaporate slowly, taking billions of years, then the core of the planet would "relax" and expand (most materials don't like to remain compressed once any pressure is removed).

But if, instead, the planet's outer layers were to be torn away far more rapidly, the core would cool suddenly. Just like a steel sword being quenched, it would harden relatively fast and retain its dense state as a "very compressed super-Earth."

Even though Grasset and his team state that they used conservative assumptions in their calculations, he also admitted that there are a lot of uncertainties still to be examined. The truth is, no one really knows how materials like rock and metal react to the extreme pressures found inside gas giants.

While this may be the best hypothesis we have to explain how super-dense planets like these form, it has to remain – at least for now – only a hypothesis.

Image: An artist's impression of a small superdense planet with its nearby parent star in the background. Credit: Markus Hammonds/

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.

The Transit

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.

Hot Jupiters

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.

Habitable Worlds

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.

A Phantom

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.

Keeping Warm

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.

Crazy Aurorae

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.

The "Candidates"

"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.

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.


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.

Let's Go!

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.