Sunday, October 21, 2012

The Strange Case Of Fomalhaut b

Some exoplanets just refuse to go gentle into that good night! Fomalhaut b is such a planet--a dying and rising remote world that refuses to stay dead. It is an object that was at first declared to be a planet, but was later determined not to be a planet--until it was again designated a planet in October 2012!

Astronomers have been searching for planets orbiting stars beyond our own Sun for centuries. The Dutch astronomer, physicist, and mathematician, Christiaan Huygens (1629-1695), carried out the first known search for exoplanets hundreds of years ago. Unfortunately, the next few centuries were richly marred by false alarms and dashed hopes. But, at last, on October 6, 1995, Dr. Michel Mayor and Dr. Didier Queloz of the Geneva Observatory in Switzerland, made the historic announcement in the journal Nature that they had indeed discovered the very first exoplanet orbiting a normal Sun-like star, 51 Pegasi. Since then, hundreds of other exoplanets have been spotted and later confirmed by dedicated planet-hunters. Most of the exoplanets discovered so far have been found by astronomers using the radial velocity method (Doppler shift method), that searches for a tattletale wobble in a parent star, indicating that there is a planet circling it, and tugging on it gravitationally. The radial velocity method favors the detection of extremely massive planets, like the gas-giants of our own Solar System, Jupiter and Saturn. However, the method also favors the discovery of planets orbiting their stars in fast, close orbits which means that, unlike Jupiter and Saturn that whirl around our Sun in distant orbits, the massive gas-giant exoplanets discovered are at roasting distance from their fiery stellar parents. These massive gas-giant planets that orbit close to their parent stars are called hot Jupiters and, until 51 Pegasi b was discovered, astronomers did not believe that such weird worlds could possibly exist--and that gas-giants could only form in orbits much more distant from the ovens of their parent stars.

Many astronomers were bewildered by the this discovery. The then-new observations suggested a planet as hefty as Jupiter, circling very close to its parent star 51 Pegasi (51 Peg, for short)--dwelling in the constellation Pegasus. 51 Peg b is roughly 4,300,000 miles away from its star--a very tiny fraction of the distance between our Sun and Mercury--the innermost planet in our Solar System. It orbits 51 Peg every 4.2 days.

What was the hefty 51 Peg b doing so close to its stellar parent? How could this newly discovered faraway world even survive in its weird and hellish orbit? Within days, however, other astronomers confirmed the Mayor and Queloz discovery, and several teams of astrophysicists tested out the possibility for such a planet's existence by using computer models. To their surprise, the calculations suggested that a planet such as 51 Peg b could indeed easily survive the extreme radiation pouring out from its parent star, and would likely shed only a very small amount of its mass during the billions of years both it and its stellar parent dwelled together in such a tight gravitational embrace.

Of the hundreds of exoplanets discovered since 51 Peg b some have proven to be extremely bizarre beasts dwelling in the cosmic zoo, while others are hauntingly similar to the familiar planets in our own Solar System. However, there has never been a discovery--at least, not yet--like the exoplanet Fomalhaut b.

Fomalhaut is one of the brightest stars in the sky. It is also relatively close to Earth--a "mere" 25 light-years away in the constellation Piscis Austrinus. One light-year is equivalent to the distance that light can travel in a vacuum in one year--which is 5,880,000,000,000 miles. This brilliantly incandescent nearby star has captured the attention of astronomers for a very long time. In 2008, astronomers who had been observing this bewitching star, using the venerable Hubble Space Telescope (HST), announced that they had spotted a planet circling it. The planet, Fomalhaut b, was shrouded by a heavy veil of obscuring dust as it circled its stellar parent. In fact, the planet was whirling around its star from within a vast debris ring that was surrounding, but slightly offset from, Fomalhaut. Based on Fomalhaut b's mass (originally thought to be about three times that of Jupiter), as well as where it is situated, astronomers suggested that its gravitational pull probably explained the debris ring's characteristics. Dr. Paul Kalas of the University of California at Berkeley, one of the original discoverers of Fomalhaut b, told the press on November 13, 2008 that "The gravity of Fomalhaut b is the key reason that the vast dust belt surrounding Fomalhaut is cleanly sculpted into a ring and offset by the star." The sharp edge and off-center belt suggested to Kalas that a planet in an elliptical (football-shaped) orbit around the star was shaping the inner edge of the belt, in a way very similar to how the moons of Saturn shape the edges of its rings. Fomalhaut b also had the distinction of being the first exoplanet to be imaged in a visible-light snapshot! Kalas added in 2008 that "It's a profound and overwhelming experience to lay eyes on a planet never before seen."

The strange case of Fomalhaut b commenced when some astronomers began to question the object's planetary status. These scientists suggested that Fomalhaut b, far from being a long-lived planet, was in reality merely a short-lived dust cloud--and they cited brightness variations reported by the discovery team, as well as the disquieting fact that NASA's infrared Spitzer Space Telescope was unable to resolve its infrared signature, as strong clues indicating that Fomalhaut b was merely a dust cloud circling the brilliant star.

The original study that had determined Fomalhaut b was a planet reported that its brightness varied by roughly a factor of two, and claimed that this was evidence that it was a planet accreting gas. However, the skeptical astronomers said this really indicated that the mysterious object was merely a transient dust cloud.

Debate flourished for many years over Fomalhaut b's true identity. But, in October 2012, after much heated controversy, Fomalhaut b soared, like the Phoenix bird rising, back up to true exoplanet status. NASA finally determined that the original theory was correct.

"Although our results seriously challenge the original discovery paper, they do so in a way that actually makes the object's interpretation much cleaner and leaves intact the core conclusion, that Fomalhaut b is indeed a massive planet," said Dr. Thayne Currie to the press on October 26, 2012. Currie, one of the authors of the new paper, is now at the University of Toronto. This second study, bouncing Fomalhaut b back up into the pantheon of exoplanets,was developed by NASA scientists after they had taken a second peek at the Hubble data.

Currie and his team reexamined the Hubble observations of the star dating from 2004 to 2006, and discovered that the hotly disputed exoplanet was easily seen at visible wavelengths. They also made a new detection in violet light. In contrast to the earlier team's findings, Currie's team found that the exoplanet maintained a constant brightness. However the second team was also unable to spot Fomalhaut b in the infrared using the Subaru Telescope in Hawaii, probably because the exoplanet must really have less than twice the mass of Jupiter.

In addition, Currie's team claims that they have also settled the disputed issue pertaining to the exoplanet's orbit around its star. Fomalhaut b is traveling at a speed and direction consistent with the Kalas team's idea that its gravity is shaping the ring.

"What we've seen from our analysis is that the object's minimum distance from the disk has hardly changed at all in two years, which is a good sign that it's in a nice ring-sculpting orbit," said Timothy Rodigas to the press on October 26, 2012. Rodigas is a graduate student at the University of Arizona.

Furthermore, near Fomalhaut's ring, orbital dynamics should completely dissolve a compact dust cloud in as little as 60,000 years.

Fomalhaut is about 200 million years old and will likely burn out in about a billion years. It is a much more short-lived star than our Sun, which is about 4.5 billion years old, and will not burn out for another 5 billion years. Kalas commented to the press back in 2008 that "Fomalhaut b is surrounded by a planetary ring system so vast it would make Saturn's rings look pocket-sized by comparison. Fomalhaut b may actually show us what Jupiter and Saturn resembled when the Solar System was about a hundred million years old."

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.

No comments:

Post a Comment