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Exotic Exoplanets Tour

There are over a thousand known planets, all but eight of them orbiting suns other than our own. They're called extrasolar planets or exoplanets and some of them are mind-boggling. Here are half a dozen exotic specimens.

The first hot Jupiter
Two Swiss astronomers made history in 1995 when they discovered the first exoplanet orbiting a Sun-like star: 51 Pegasi b.

Exoplanets are named using a lower case letter after the star's name. 51 Pegasi is the star and “b” is for the first planet discovered. If someone finds another one, it will be 51 Pegasi c. However 51 Pegasi b is also unofficially known as Bellerophon after the legendary Greek hero who rode Pegasus the winged horse.

In the Solar System, planets near the Sun are small and rocky; the large gas planets are farther out. So astronomers were taken aback by a giant planet circling its star in just over four Earth days! Even Mercury is far enough from the Sun to take 88 days.

At first, everyone assumed that the new planet was some sort of monster rocky planet, only later realizing that it was a gas giant. Bellerophon was the first of the planets known as hot Jupiters.

Hot Jupiters are tidally locked to their stars, so the same side always faces the star, just as the same side of the Moon always faces Earth. For hot Jupiters this means that one side is much hotter than the other. Our own Jupiter has a fairly even temperature.

Yet hot Jupiters aren’t all alike. Here are two quite different ones.

Space ember
One of the hottest and blackest planets known is HD 149026 b. It orbits the star numbered 140926 in the Henry Draper (HD) catalog.

Gas giants are mostly hydrogen and helium. However HD 149026 b not only contains heavier elements, but also has a large dense core. The temperature is over 2000 oC (3700 oF) on the sunny side, hot enough to melt iron and boil silicon. The atmosphere probably consists of metal oxides like vanadium and titanium.

The opaque upper layers absorb almost all of its star's light, reflecting nothing back. This would make the planet blacker than coal, but it radiates heat, so HD 149062 b would glow like a dying ember.

It's a corker
Although HD 149026 b is dense, HAT P-1 b would bob like a cork if there were a big enough cosmic pool. It was discovered by HAT, a robotic telescope network monitoring the sky for transits - the dimming of a star when a planet passes in front of it. Its diameter is over 170,000 km (107,000 miles).

Interestingly, HAT P-1 b orbits a star in a double star system.

It rains rock
CoRoT-7 b, discovered by space telescope CoRoT, is a rocky planet, but it's not like home. It's much larger than Earth, so is a "super-earth." Like the hot Jupiters, it's much closer to its star than Mercury is to the Sun, and is tidally locked.

CoRoT-7 b's shady side is -187 o C (-369 oF), but the sunny side is 2300 oC (4200 oF), hot enough to melt rock. Its surface would have lava oceans. And its atmosphere would be vaporized rock, so when it rains, the atmosphere would condense into pebbles, not raindrops.

Not all exoplanets are big and hot, but these are the easiest to find. (See “Searching for Extrasolar Planets” to learn more.) However the Kepler mission data is expected to include smaller planets in the habitable zone of their stars. Kepler is a space telescope which has been hunting for habitable planets. The habitable zone is the area in which water could exist as a liquid.

In the Star Wars films Luke Skywalker's home planet Tatooine has a double sun.

Scientists argued about whether binary stars could have planets, because of the complex gravitational forces. The argument was settled by finding planets such as HAT P-1 b in binary systems. But the first exoplanet found orbiting both of its suns is Kepler-16 (AB) b.

Kepler-16 (AB) b isn't snuggled up next to a star. It completes an orbit every 229 Earth days, similar to Venus's orbit. Both stars are cooler than ours and the planet is just beyond the habitable zone, making it a chilly -100 oC (-150 oF).

You couldn't stand on Kepler-16 (AB) b to watch the suns set, because it's a gas planet similar to Saturn. Nonetheless George Lucas may have been pleased by its discovery in 2011.

Planet-sized diamond
To finish, here's something completely different.

The very first extrasolar planets discovered were orbiting a pulsar, which was a bigger shock than the hot Jupiters that followed. A pulsar is a spinning neutron star which gives out strong radio signals. It's the end product of a supernova explosion, so no one expected planets. Yet a number of pulsars are now known to have planets.

Pulsar PSR J1719-1438 and its planet PSR J1719-1438 b were once a double star system. When the massive star exploded as a supernova, the two stars stayed separate instead of merging. However the pulsar's gravity stripped its companion of everything except its carbon core. On Earth carbon in the Earth's interior, under high pressure, forms diamonds. Astronomers think this has happened to the former star. A planet-sized diamond! The ultimate bling.

Kepler Home Page, http://kepler.nasa.gov/
ESA Space Science – CoRoT, http://www.esa.int/science/corot
The Extrasolar Planets Encyclopaedia, http://exoplanet.eu/

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