Thursday, October 20, 2011

Planet Hunting Through the Rings of Saturn

Planets are formed from when clouds of dust circling a star start to clump together and coalesce into a planet. At the recent Signposts of Planets conference held at the Goddard Space Flight Center, astronomers released the first photo of what is likely a planet forming around a distant star. Because the star is many light years away, the gestating planet only shows up as a smudge in the photograph, so it’s hard to see its finer details, or of its surrounding dust cloud.

In fact over 100 dust clouds have been found circling around far away stars, and many of them likely have planets hiding some were inside them too. Astronomers have been scrutinizing these dust clouds to better understand how planets form inside of them, but it’s hard to get much resolution on these tiny spots of light.

Fortunately though, we have an ideal model right here in our own backyard; Saturn. Really, on the cosmic scale, it’s pretty much in our living rooms.

Saturn’s rings are made up of tiny ice particles swirling around the planet. They’re a perfect small scale model of what happens around stars when planets start to form. Well almost perfect, it applies to colder dust clouds that start forming about as far away from a star as our asteroid belt is from the Sun. , but it’s really good model of them. In addition, the Cassini space craft has been in orbit around the ringed gas giant since 2004, sending back the most complete, detailed pictures ever seen of its rings.

Saturn’s rings are complicated and full of bands, gaps and ripples. Sometimes there are tiny moons hiding in the gaps clearing away debris while they orbit, sometimes not. Figuring out why the gaps exist and how they’re formed helps astrophysicists be able to look at similar dust rings around faraway stars and tease out possible planets by looking at how these gaps behave.

How do scientists know that the rings of Saturn actually have anything to do with dust clouds around stars? Well observations of distant stars have given hints that the structures of some of the stellar clouds resemble the rings of Saturn in important ways. If a star has a cloud of gas and dust orbiting around it almost edge on to Earth, the cloud will pass in front of the star, eclipsing it. Observers on Earth will see the light dim and by monitor its light levels, they can find structure in its rings.

Eric Mamajek observed in this kind of dimming around star J1407 between 2001 through 2009. The star darkened then brightened again in discrete jumps, exactly as if a stellar sized version of Saturn’s banded rings passed between the star and Earth.

“My feeling is that this discovery will be one of the many that will be made in the next few years,” said Alice Quillen, a professor of astrophysics at the University of Rochester and co-author of the paper about J1407.

The Cassini satellite has been orbiting Saturn for seven years, and it’s been beaming back amazing images that tell scientists a lot about how to find moons hidden amongst its rings. The easiest way is to look for gaps, where the dust has been cleared out by orbiting moons. Near the outer edge is the Keeler gap, a thin band that’s been cleared of dust. Orbiting inside it is the tiny moon Daphnis. Farther in is the Enke gap, with another tiny moon, named Pan clearing its way.

Bewilderingly, there are gaps that don’t have moons inside of them but are still the result of moons. One of the gaps, the Huygens gap, is a pretty big one about two-thirds of the way out but there’s nothing inside of it to plow debris away. What astronomers realized is that this gap lines up with the resonance of a moon called Mimas (which looks like the Death Star from Star Wars).

Resonance is when the orbits of two satellites, planets, moons or whatever evenly line up at the same relative point around the planet. Think of looking down at two moons lined up with each other orbiting a planet. When the outer one makes one complete orbit and returns to its original position, the inner one with less distance to cover, has orbited the planet exactly twice, and is also at the same position and again lined up with its companion.

The dust particles that would be in the Huygens gap were lined up with one of the resonances of Mimas. However over eons, the regular gravitational pull of the moon pulled all the dust out that orbit leaving a clear gap. The more astronomers understand the gaps in the rings and how nearby moons shape them, the better they can extrapolate where planets might hide near gaps in a star’s dust cloud.

There are also formations inside of the rings themselves that are small scale signs of the formation of moonlets. Dubbed “propellers” these thin wispy structures inside the ring are where dust and particles swirl around and fall into each other just the way planets around stars form.

“Those things are telling you ‘Hello, there is a planet here!”” said Aurelien Crida, an astronomer at the Observatoire de la Côte d'Azur.

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