Searching for planets beyond our solar system is a bit like playing Goldilocks — we keep looking for that one that will be just right to host life. While astronomers haven't found a perfect fit yet, they have found plenty that are too big, too hot, too cold, too dense, too close to their star, or too distant.

The first exoplanet discovery was in 1988, though it was controversial at the time and wasn't officially confirmed until 2003. Over the years, more than 330 extrasolar planets have been found, nearly all of them using indirect methods such as detecting the wobble of a star due to the gravitational pull of an orbiting planet or the slight dimming of the star's light as a planet passes in front of it.

Another method uses an intervening star to gravitationally magnify the light from a more distant star with planets. And in a few rare cases, the conditions are right for direct imaging of an exoplanet using a technique that allows powerful ground-based telescopes to separate the faint light of a planet from the overwhelming light of its star.

Of the planets discovered beyond our solar system to date, here are five of the most extreme:

5. CoKu Tau/4

This exoplanet, found about 420 light-years away around the star CoKu Tau 4, is one of the youngest extraterrestrial worlds discovered. Its star is thought to be only about 1 million years old, meaning its planet must be even younger. That makes it a wee young 'un next to the grizzled Earth, which is about 4.5 billion years old. Studying baby planetlets like this could help astronomers figure out how planets are born in the first place. (Illustration, above: NASA/JPL-Caltech/R. Hurt (SSC-Caltech))

4. PSR B1620-26 b

This bizarre world, estimated to be 13 billion years old (almost three times the age of Earth), is one of the oldest known exoplanets. Its age isn't its only weird factor, though. The gas-giant planet, which lies in the globular cluster M4, appears to be orbiting around not one, but two stars, in a binary system. Its parent stars are a small, dense white dwarf star and a quickly rotating pulsar. (Illustration: NASA and G. Bacon (STScI))

3. SWEEPS-10

This zippy planet circles its parent star at one of the fastest rates yet discovered — about once every 10 hours. It accomplishes this feat by dwelling extremely close to its star — only about 1.2 million kilometers (745,000 miles), or roughly three times the span between the Earth and the moon. At such a distance, the planet needs all of its hefty bulk (about 1.6 times the mass of Jupiter) to resist being torn apart by its star. (Illustration: NASA, ESA, A. Schaller (for STScI))

2. OGLE-2005-BLG-390Lb

In contrast to most exoplanets, which are fiery balls of gas, this globe is thought to be hard and rocky. The planet, about 5.5 times as massive as Earth, is still one of the coldest and most Earth-like planets found to date. Sadly, though, with a surface temperature of -364 degrees Fahrenheit (-220 degrees Celsius) it is probably too frigid to host life. (Illustration: NASA, ESA and G. Bacon (STScI))

1. HD 149026b

This boiling world is one of the hottest and densest ever found. None too pleasant to visit, the surface of the planet is about 3700 degrees Fahrenheit (more than 2,000 degrees Celsius) — about three times hotter than the surface of Venus, the hottest planet in our solar system. HD 149026b is so hot that scientists think it absorbs almost all of the heat from its star, and reflects almost no light. The scorching ball is smaller than Saturn, but has a core that weighs 70 to 90 times the mass of the entire Earth. (Illustration: NASA/JPL-Caltech)

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