Gliese 581 C marked a milestone in the search for worlds beyond our solar system. It is the smallest exoplanet ever detected, and the first to lie within the habitable zone of its parent star, thus raising the possibility that its surface could sustain liquid water, or even life. It is 50 percent bigger and 5 times more massive than Earth.

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HD 189733b was among the first planets to have its air “sniffed”. By analyzing light from the star-planet system, astronomers determined the planet’s atmosphere contains thick clouds of silicates similar to grains of sand. Curiously, no water vapor was detected, but scientists suspect it is hidden beneath the clouds.

A year on HD209458b is only 3.5 Earth-days long. The planet orbits so close to its star that its atmosphere is being blown away by gales of stellar wind. Scientists estimate the planet is losing at least 10,000 tons of material every second. Eventually, only a dead core of the shrinking planet will remain.

The oldest known planet is a primeval world 12.7 billion years old that formed more than 8 billion years before Earth and only 2 billion years after the Big Bang. The discovery suggested planets are very common in the universe and raised the prospect that life began far sooner than most scientists ever imagined.

The youngest exoplanet yet discovered is less than one million years old and orbits Coku Tau 4, a star 420 light-years away. Astronomers inferred the planet’s presence from an enormous hole in the dusty disk that girdles the star. The hole is 10 times the size of Earth’s orbit around the Sun and probably caused by the planet clearing a space in the dust as it orbits the star.

Upsilon Andromeda b is tidally locked to its sun like the Moon is to Earth, so one side of the planet is always facing its star. This setup creates one of the largest temperature differences astronomers have ever seen on an exoplanet. One side of the planet is always hot as lava, while the other is chilled possibly below freezing.

SWEEPS-10 orbits its parent star from a distance of only 740,000 miles, so close that one year on the planet happens every 10 hours. The exoplanet belongs to a new class of zippy exoplanets called ultra-short-period planets (USPPs), which have orbits of less than a day.

There are known exoplanets that have one, two and even three suns. But one bizarre class of planet-sized objects has no suns at all, and instead floats untethered through space. Called “planemos,” the objects are similar to, but smaller than, brown dwarfs, failed stars too small to achieve stellar ignition.

Epsilon Eridani b orbits an orange Sun-like star only 10.5 light years away from Earth. It is so close to us telescopes might soon be able to photograph it. It orbits too far away from its star to support liquid water or life as we know it, but scientists predict there are other stars in the system that might be good candidates for alien life.

51 Pegasi b was the first planet discovered in orbit around a normal star other than our Sun. The planet, a hot Jupiter, also goes by the moniker “Bellerphon,” after the Greek hero who tamed winged-horse Pegasus, in reference to the constellation Pegasus where the planet is located.