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Exoplanets

The planets are rarely seen directly, but are found due to a dip in the brightness of a star as a large planet crosses in front of it. When the dip in brightness happens at regular intervals, the conclusion is that it is a planet circling that star. The light of the star shining through that planet's atmosphere tells us, via spectroscopes, what that atmosphere is composed of. The sort of planet we have discovered most frequently is the type large enough and close enough to its sun to block off a noticeable amount of light. We have already found several thousand planets, and there are probably billions more. The ones we have found are all close to us, astronomically speaking.

The illustration above shows much more clearly than we actually see of what happens. But this does show why it happens and why astronomers conclude a planet is crossing the face of the star when the light dims at regular intervals. A change in what the spectroscope picks up also tells us that there is something else there besides the star itself.

In addition, there are a number of confirmed planets which dwarf Jupiter.

Chthonioan Planets: these planets are found very close to their stars. It is thought they used to be very large "super Jupiters" which, because of their close proximity to their stars, have had their atmospheres stripped away, leaving only a rocky or metallic core. This stripping away is called "hydrodynamic escape." It is what happened on our moon and on Mercury if they ever had atmospheres at all.

HD 209458 b is an example of a planet that is in the process of having its atmosphere stripped away, though it is not yet a chthonian planet nor will it become one soon.
COROT-7b is the first chthonian planet discovered.

Artist’s conception of the Corot 7b system

Hot Jupiters:

These are massive gas giants with about the same mass as Jupiter. They orbit close to a star so they have a high temperature, whereas our Jupiter is very cold. Their orbit radius ranges out to about 0.5 AU whereas Jupiter is at about 5 AU. Because they are close to their parent star they are easily discovered as they transit the star, dimming its light.

• Most of these have nearly circular orbits (low eccentricities). This is because their orbits have been circularized, or are being circularized, by the tidal pull of the star. This also causes the planet to synchronize its rotation and orbital periods, so it always presents the same face to its parent star — this is called "tidal locking."
• They exhibit high-speed winds distributing the heat from the day side to the night side, thus the temperature difference between the two sides is relatively low.
• They are more common around yellow stars and somewhat less common around orange stars. Hot Jupiters around red dwarfs are very rare.
• It has been found that several hot Jupiters have retrograde orbits, meaning their orbit is going in a different direction from their star's spin.
•  By combining new observations with the old data it was found that more than half of all the hot Jupiters studied have orbits that are misaligned with the rotation axis of their parent stars, and six exoplanets in this study have retrograde motion.
• Hot Jupiter’s with orbit periods of half a day around their star occur with stars of mass less than 1.25 solar masses. These planets do not accord with standard theory.

Hot Saturns or “Puffy Planets”:

The density of Saturn is less than the density of water. That is the same way with the "hot Saturns." Their density is out of proportion to their size. They are gas giants with a large radius, but that large radius appears to be the result of a combination of a close orbit to their stars along with intense internal heating making them "puff" or inflate the atmosphere of the planet. So far six of them have been discovered.

Hot Neptunes:

These exoplanets are called "hot Neptunes" because they have the approximate mass of Uranus or Neptune. They orbit close to their stars, usually closer than Earth is to our sun. It is suspected that there are a great number of them out there, yet to be discovered.

Super Neptunes:

Astronomers have discovered planets somewhat larger and more massive than Neptune. One orbits a star 120 light-years from Earth. Neptune has a diameter 3.8 times that of Earth and a mass 17 times Earth's. The best example of a "Super Neptune" is named HAT-P-11b, and is 4.7 times the size of Earth and has 25 Earth masses. An illustration of this system is below.

HAT-P-11b in Cygnus: This artist's conception reveals the newly discovered Super-Neptune planet orbiting a star 120 light years away from Earth. Normally blue in color, its red hue is caused by the illumination from the nearby Red Dwarf star. The small rock represents the possibility of a moon circling the exoplanet.

Water Worlds or Ocean Planets:

These are planets which have a lot of water. The surface of these planets may have oceans of water hundreds of miles deep. If the planet is close enough to its star for the water to reach the boiling point, it could become supercritical. What this means is that the steam evaporating from the water would be, essentially, as thick as the water and would have an even greater dissolving power. If the water planet were farther away, and thus cooler, it might have a very thick water-based atmosphere, producing a strong greenhouse effect on the planet -- a hot, steamy planet. The smaller the ocean planet, the less the gravity, and so it is assumed that not only could the water evaporate more quickly, but that the oceans themselves would have very high waves.

Super Earths:

From Wikipedia: The first super-Earth around a main sequence star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876 and received the designation Gliese 876 d (two Jupiter-sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf), it may have a surface temperature of 430–650 kelvin and may support liquid water.

Other Planets:

Considering the size of our galaxy and how little of it we have been able to explore, there may be many other planets out there the sizes of our planets. We have found "rogue planets" which appear to orbit in the galaxy independently of any star.

Brown Dwarfs

Are these planets or stars? If they shine at all, it is only in the infrared. They are too large to be a planet and too small to be a star.

April 2015, farthest exoplanet yet detected

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