Sunday, August 12, 2012

Diamond Planet

"Curiouser and curiouser," exclaimed Alice, lost in a Wonderland inhabited by a truly impressive array of oddballs. Her words can easily be echoed by today's planet-hunters who search for extrasolar planets--planets that circle stars other than our Sun--who have managed to discover a bizarre array of other"oddballs" dwelling in our Milky Way Galaxy.

Scientists have been searching for planets orbiting stars beyond than our Sun for a very long time. In the 18th century, the possibility of the existence of extrasolar planets was mentioned by Sir Isaac Newton in the General Scholium that ends his Principia. Newton, making a comparison to the Sun's own familiar family of planets, writes: "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."

Many times during the 20th century, ecstatic astronomers announced what they thought was the first sighting of a planet beyond our own Solar System. They then looked on unhappily as other astronomers failed to confirm their "discoveries". In 1992, however, one happy radio astronomer hit the elusive jackpot and announced evidence confirming the existence of two extrasolar planets circling around a dense little stellar corpse in the Milky Way.

Astronomer Dr. Alexander Wolszczan of Pennsylvania State University made his announcement after observing radio emissions from a compact millisecond pulsar located about 1,300 light-years from Earth. One light-year is the distance that light can travel in a vacuum in one year--5,880,000,000,000 miles!

The pulsar, known by the bland name of PSR B1257 + 12, is a tiny dense denizen of the Virgo constellation. A pulsar is a little ball, perhaps about 12 to 20 miles in diameter, in which the collapsed core of a massive star, containing up to 1,000,000,000 tons of matter, literally is squeezed to the size of a city on Earth. A pulsar is a spinning neutron star--the relic core of a massive star that has died in a spectacular supernova blast--and these exotic objects have a density that is equivalent to 1,000,000 times that of the density of water.

It was later determined that PSR B1257 + 12 is orbited by several planets--and they are true "oddballs". They are probably rocky bodies, like the Earth, but that is where all resemblance ends. Pulsar planets, unlike Earth, can have no atmosphere. They are extremely unpleasant worlds, showered mercilessly by deadly radiation.

The vicinity of a pulsar was about the last place astronomers expected to observe planets. Such oddities should have tipped astronomers off to the existence of many, many more "oddballs" to come.

And, come they certainly did! Although the pulsar planets were the first extrasolar planets to be discovered, astronomers still sought the "Holy Grail" of planets circling a normal "main-sequence" (hydrogen-burning) star like our Sun. Triumph came in 1995, when astronomers Dr. Michel Mayor and Dr. Didier Queloz of Switzerland's Geneva Observatory announced the first convincing evidence of an extrasolar planet circling a normal Sun-like star dwelling outside of our own Solar System. However, the newly discovered extrasolar planet turned out to be a true "oddball" because it was as hefty as Jupiter--the largest planet in our Solar System--but it circled its star at a mere fraction of the distance between Mercury and the Sun. The star that hosts the roasting planet is dubbed 51 Pegasi, and the strange planet was suitably named 51 Pegasi b. 51 Pegasi b was the first extrasolar planet spotted belonging to a new class of objects termed "hot Jupiters"--giant gaseous planets orbiting fast and close around their parent stars. So far, most of the extrasolar planets discovered have been "hot Jupiters". This is because most of the extrasolar planets were discovered using the Doppler (radial velocity) method, which favors the discovery of giant planets that orbit fast and close to their fiery parent stars. However, there are other methods, in addition to the radial velocity method, that are now being used. Those other methods are able to spot smaller worlds that twirl around their stars in more distant orbits. For example, planet-hunters are now discovering transiting extrasolar planets, which are planets that float directly in front of the face of their star. Also, gravitational lensing techniques are currently being used to discover extrasolar worlds. Gravitational lensing is a prediction of Albert Einstein's General Relativity whereby a large, foreground celestial object bends, or distorts, the light emitted by a more distant object.

Since those initial discoveries in the mid 1990s, astronomers have discovered planetary systems akin to our own Solar System, as well as increasingly smaller and smaller planets--planets the size of our own Solar System's Uranus and Neptune. Planet-hunters have now succeeded in spotting much smaller planets approaching our own Earth in size. With decreasing size, astronomers expected to discover increasingly Earth-like worlds. While this may indeed be the case in many instances, smaller worlds have proven that they can be just as odd as many of the much larger extrasolar planets observed so far.

Super-Earths are bizarre extrasolar planets that are unlike any that dwell in our own Solar System. They are smaller than the familiar four giant planets that circle our Sun--even Neptune, which is our Solar System's smallest giant planet. But Super-Earths are more massive than our own Earth, and they can be composed of rock or gas or both! The extrasolar planet 55 Cancri e was discovered orbiting a nearby star in our Milky Way Galaxy, and it is a very dark, carbon-rich, rocky world. In October 2012, amazed astronomers announced that at least one-third of this "oddball's" mass is composed of diamond!

55 Cancri e has a radius about twice that of our planet, and its mass is eight times greater. It orbits the nearby Sun-like star, 55 Cancri, which is located about 40 light-years from Earth in the constellation Cancer. This "oddball" world is one of a family of five planets circling that star, whipping around it at breathtaking speed. 55 Cancri e circles its star in a mere 18 hours--in marked contrast to Earth's year which is 365 days long! The planet is searing-hot, with a sizzling temperature of 3,900 degrees Fahrenheit. Such a hostile world is not likely to harbor delicate living creatures.

This bizarre planet was first observed transiting its parent star in 2011. This enabled astronomers to measure its radius. This newly acquired information, combined with an estimation of the planet's mass, allowed Dr. Nikku Madhusudhan, a Yale University postdoctoral researcher, and his colleagues, to determine its chemical composition. The researchers accomplished this feat by using models of the planet's interior and by computing all possible combinations of elements and compounds that could yield those characteristics.

"This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth. The surface of this planet is likely covered in graphite and diamond rather than water and granite," Madhusudhan noted in the October 11, 2012 Yale News.

Astronomers had earlier reported that 55 Cancri contained more carbon than oxygen. Dr. Madhusudhan and his coworkers went on to confirm that large quantities of carbon and silicon carbide, as well as a tiny quantity of water ice, were available when 55 Cancri e was in the process of forming.

Astronomers had also suggested previously that 55 Cancri e contained a very large quantity of super-heated water--basing this on the assumption that the "oddball" world possessed a chemical composition similar to that of Earth.

However, the new research indicates that the planet really has no water at all! In fact, 55 Cancri e is apparently composed mostly of carbon--in the form of diamond and graphite, iron, silicon carbide, and, perhaps, silicates. The study further indicates that at least a third of the "oddball's" mass--which is equivalent to that of three Earths--could be diamond!

The identification of a carbon-rich Super-Earth indicates that remote rocky planets circling stars beyond our Sun can no longer be assumed to have interiors, atmospheres, chemical constituents, or biologies akin to those of our own planet, Madhhusudhan continued to explain. This discovery also opens new vistas for the study of geophysical processes and geochemistry in Earth-sized distant worlds. A carbon-rich composition could play a starring-role in a planet's plate tectonics and thermal evolution--with strong implications for mountain formation, volcanism, and seismic activity.

Dr. David Spergel, professor of astronomy and chair of astrophysical sciences at Princeton University, who was not a co-author of the study, noted in the October 11, 2012 Yale News that "Stars are simple--given a star's mass and age, you know its basic structure and history. Planets are much more complex. This 'diamond-rich-super-Earth' is likely just one example of the rich sets of discoveries that await us as we begin to explore planets around nearby stars."

This new research represents the first time that astronomers have spotted a likely diamond planet around a Sun-like star and specified its chemical composition.

The paper is titled "A Possible Carbon-rich Interior in Super-Earth 55 Cancri e", and has been accepted for publication in the journal Astrophysical Journal Letters. The authors of the paper are Madhusudhan, Yale University geophysicist Dr. Kanani Lee, and Dr. Olivier Mousis, who is a planetary scientist at the Institut de Recherche en Astrophysique in Toulouse, France.

I am a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. Although I have written on a variety of topics, I particularly love writing about astronomy because it gives me the opportunity to communicate to others the many wonders of my field. My first book, "Wisps, Ashes, and Smoke," will be published soon.

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