The world was first introduced to the sandswept, imaginary desert planet Tatooine back in 1977, when it made its debut in the very first Star Wars movie. This strange, fictitious world was the home of Luke Skywalker, among others. Adding to the generally bizarre nature of this dry and sandblasted planet, was the fact that it had not only one sun, but two!
Astronomers once thought that such circumbinary solar systems, where worlds twirl around two stars, might not be able to form in the real Universe because they would be too unstable–although there were some tantalizing hints that they might exist. But astronomers now know that Tatooine-like worlds really do dwell in our Milky Way Galaxy.
Indeed, astronomers are coming to the realization that planet formation is a very common occurrence. Back in 1977, no one had, as yet, spotted worlds circling stars beyond our own Sun. The first so-called extrasolar planet was not discovered until 1995–but now the number of such worlds known to astronomers is staggering, far exceeding the eight familiar major planets in our own Solar System. Some scientists even calculate that there are more planets in our Galaxy than there are stars! Extrasolar planets exist in huge numbers, and with great diversity–some are eerily familiar worlds, hauntingly resembling the planets in our own Solar System. Others, however, are unlike anything astronomers once believed could exist.
The search for planets around other stars was a difficult quest, and their ultimate discovery arguably represents one of humanity’s greatest triumphs. Discovering a planet the size of Jupiter circling a remote star in our Galaxy has been compared to observing light bouncing off a speck of dust close to a 1,000-watt light bulb when the observer is miles away. Even so-called “nearby” stars are extremely remote, and in visible light, a planet glows only in reflection. Indeed, the distance to even the nearest star beyond our Sun is almost unimaginably great. The nearest star to our Sun is actually a triple star system, Alpha Centauri, and it is situated about four light-years from Earth–or about 24 trillion miles.
Astronomers have been searching for planets circling stars beyond our Sun for a very long time. The Dutch astronomer, physicist, and mathematician, Christiaan Huygens (1629-1695) undertook the first known search for extrasolar planets in the 17th century. Alas, the next few centuries were tarnished with false alarms and one disappointment following on the heels of another. It was not until 1988–eleven years after the first Star Wars movie was released–that the first true hints of triumph began to emerge. For example, Dr. Gordon A.H. Walker and his colleagues at the University of British Columbia in Canada cautiously reported signs of unseen planetary-mass objects orbiting a handful of nearby stars. These findings were reported in an extremely cautious manner, however, with disclaimers stating that orbiting planets were just one of many possible interpretations of the data.
In 1989, Dr. David W. Latham of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and four colleagues, announced clear evidence for what might be a planet orbiting an obscure star known by the bland name of HD 114762. But because Latham’s planet was richly endowed with more than 10 times the mass of Jupiter, astronomers tended to regard the object as either a low-mass normal star or an interesting little twerp of a substellar object called a Brown Dwarf. Brown Dwarfs probably are born just like normal stars, but because they are much less massive, their nuclear-fusing furnaces never catch fire.
Theorists generally agree that it is not especially difficult for planets to form around a common Sun-like star. As a dense knot within a cold, dark interstellar molecular cloud collapses to form a new star, it usually leaves behind a disk of dust particles circling this so-called protostar. These particles readily stick together to create increasingly larger and larger objects that eventually grow into full-fledged planets. The formation and evolution of our own Solar System, for example, is believed to have started about 4.6 billion years ago with the gravitational collapse of a minute segment of a giant, frigid, dark molecular cloud. Most of the collapsing mass congregated at the center, forming our Sun, while the rest flattened into a protoplanetary disk from which planets, moons, asteroids, comets, and other small Solar System bodies emerged.
Protoplanetary disks have been observed around a number of stars dwelling in young star clusters. They form around the same time the baby star is born, but at the earliest stages cannot be seen because of the impenetrable opacity of their veiling envelope. The accretion disk that nourishes the central protostar is thought to be very hot and massive. The extremely hot temperatures in the inner portion of the disk, nearest to the baby star, vaporize most of the volatile material, such as organics, water, and some rocks, leaving behind only the most refractory elements such as iron. Ice is able to survive only in the outer limits of the disk, far away from the star. Accretion disks can linger around for about 10 million years.
Tatooine-like systems, in which planets orbit two stars, which in turn orbit each other, have long been considered too unstable to exist. This is because the complex gymnastics between all of the bodies in such a system would tend to result in a large number of collisions. In addition, many of the bodies would be unceremoniously evicted from the system altogether. Astronomers have found it quite a challenge to collect data on circumbinary systems. Although they have spotted a cornucopia of solar systems beyond our own, only a few have more than one star. Mathematical simulations also have not been particularly helpful.
Despite these great difficulties, the existence of a real Tatooine-like planet was confirmed for the very first time in 2011. Astronomers using NASA’s Kepler Space Telescope, discovered a giant planet circling around a pair of binary stars that compose the Kepler-16 system, which is situated roughly 200 light-years from Earth. The extremely successful Kepler Space Telescope is aimed at a field in our Galaxy that hosts roughly 4.5 million stars, and its main goal is to discover extrasolar Earth analogs. Kepler searches for planets using the transit method. This means that it looks for tattletale dips in a star’s brightness as orbiting planets float in front of them in transit events. Kepler has the ability to detect very slight alterations in a star’s brightness. This orbiting observatory was launched in March 2009 aboard a Delta-2 rocket.
The Tatooine-like planet, named Kepler-16 (AB)b, passes in front of the faces of both of its stars, regularly dimming their light. In addition, each of the two stars eclipses its sister as they circle each other. Taken together, these gymnastics enable astronomers to calculate the radii, trajectories, and masses of all three bodies.
The planet’s distance from its stars amounts to almost three-fourths that of the distance between our planet and the Sun. It is roughly the same size as Saturn, although it is nearly 50 percent denser. This suggests that the newly discovered planet is endowed with more heavy elements. Saturn is the lightest planet in our Solar System, and it could float on water–if such a vast amount of water was available.
The study announcing the discovery was published in the September 16, 2011 issue of the journal Science. Study co-author, Dr. Alan Boss of the Carnegie Institute in Washington, told the press in September 2011 that “This discovery is stunning. Once again, what used to be science fiction has turned into reality.”
The Tatooine-like world circles on a nearly circular 229 -day orbit around its parent stars, Kepler-16A and Kepler-16B, and the sister stars are approximately 69 and 20 percent as massive as the Sun, respectively. The sister stars are very close to one another–only about one-fifth the distance between the Sun and our planet on average. This is closer than Mercury (the innermost planet in our Solar System) ever gets to the Sun. The weird sister stars complete one orbit around each other every 41 days.
The real existence of circumbinary planets had been hinted at previously. Binary stars are very common in our Galaxy. As a matter of fact, the majority of stars in our Milky Way are binaries. Lonely stars, like our Sun, are the exception, rather than the rule. Sister stars in a binary system both circle around a point in Space termed a barycenter, and astronomers sometimes noticed that these orbits were slightly altered. This suggested the existence of a planet tugging at the sister stars. However, Kepler-16 (AB)-b was the first planet that astronomers had spotted directly transiting in front of the face of its stars, temporarily diminishing their light.
In August 2012, an even weirder discovery was announced–the remarkable Kepler Space Telescope had discovered double planets orbiting double stars in the constellation Cygnus, which is 5,000 light-years distant from Earth! This enchanting discovery marks the first time multiple planets have been observed dwelling in a binary star system.
The new system, dubbed Kepler-47, hosts one Sunlike star and a little sister that is only about one-third its size. The double planets orbit around both stars in what is probably a very tricky waltz. One of the planets is slightly larger than Uranus, while the other is only three times bigger than our planet.
The new study was published in August 2012 in the journal Science. The Uranus-size outer planet is situated in the precious habitable zone, where temperatures are just right for liquid water to exist on a rocky surface. The planet also boasts a 303-day orbit that is similar to Earth’s. However, despite all of these intriguing attributes, the planet is probably an uninhabited gas-giant like Jupiter and Saturn, although its existence indicates that circumbinary planets can dwell in habitable zones.
As of September 2012, six Tatooine-like planets in all have been discovered circling double stars–and all were spotted by Kepler.
Dr. John Knoll, visual effects supervisor at the special-effects firm Industrial Light and Magic, which helped develop the Star Wars movies, commented at a NASA press conference held on September 15, 2011: “Again and again we see that the science is stranger and cooler than fiction. The very existence of these discoveries gives us cause to dream bigger, to question our assumptions.”
Tags: Planets, Tatooine
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