Exoplanets are planets that orbit a star other than our sun. The prefix exo comes from the Greek and means outside; these worlds are far, far outside our own solar system. Astronomers have confirmed more than 5,000 exoplanets orbiting distant stars. The existence of planetary systems other than our own had been surmised for centuries. But it wasn’t until 1992 that astronomers found the first two exoplanets orbiting a pulsar. Then came the confirmation of the first exoplanet orbiting a sunlike star in 1995.
Why didn’t we see them before? It’s because exoplanets are so far away, several light-years away at their closest. And it’s because – unlike stars – exoplanets don’t shine with their own light. Like Earth, they shine only with light reflected from their local stars. In contrast to their stars, exoplanets are exceedingly dim; even the largest drown in the light of their vastly brighter stars.
Before the first exoplanet discovery, most astronomers assumed exoplanets, if found, would resemble the planets in our solar system. The great shock has been that many exoplanets are far different, with their positions and orbits difficult to explain. If astronomers thought the solar system was in any way representative of other planetary systems out there in the galaxy, they’ve been disappointed. Our solar system may be the exception rather than the rule. Continue reading from EarthSky
Fifteen types of exoplanets are currently listed in the Unified Astronomy Thesaurus, and a few more names pop up in the literature. These types aren’t distinct; exoplanets can be described in more than one of these different ways. Five of the exoplanet types refer only to size (mass and diameter). In order, from smallest to biggest, they are rocky planets; super-Earths; mini-Neptunes; ice giants; and gas giants. Planets in the super-Earth and mini-Neptune size range may be ocean planets, having liquid-water oceans hundreds of kilometers deep. Some rocky planets in especially carbon-rich systems may be ultra-dry carbon planets, having rocks formed of compounds of silicon and carbon, different from our solar system’s silicon-and-oxygen rocks. Continue reading from the Planetary Society
Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula—a spinning, swirling disk of material.
At the center, gravity pulled more and more material in. Eventually the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. With that, our Sun was born, and it eventually amassed more than 99 percent of the available matter. Continue reading from NASA
