Epsilon Eridani (Epsilon Eridani b discovered August 7, 2000)
It’s actually a system of planets, not unlike how we like to call our own solar system. The name “Epsilon Eridani” stands for the parent star, or their “sun,” and it has two probable planets orbiting it: one confirmed (Epsilon Eridani b) another yet unconfirmed (Epsilon Eridani c), making it the closest planetary system at just over 10 light years from the solar system. It even has not one but two asteroid belts, an inner one between Epsilon Eridani b and the star and an outer one between b and c, and also a dust ring beyond c’s orbit believed to be produced by extrasolar comets bumping into each other.
4. Most Suns
91 Aquarii b (November 16, 2003)
When we think of planets we are used to the idea of planets around the sun in a single star system but in fact a surprising number (about one in two) of the stars we see in the night sky are really multiple star systems; that is, a group of two or more stars orbiting around their common center of mass (they appear as single points of light because of sheer distance). In the 91 Aquarii system there are five stars, and in November 2003 it was discovered that a gas giant planet was orbiting the primary star, 91 Aquarii A, and thus the planet is alternatively named 91 Aquarii Ab to distinguish it from the other stars in the system and to make room for possible undiscovered planets around those other stars. This gas giant is special because our methods of detecting exoplanets around stars require a great deal of precision which could unfortunately be thrown off by even one other nearby star, making it one of the few planets discovered in multiple star systems.
3. Most Exoplanets in One System
55 Cancri (55 Cancri b discovered April 12, 2006)
This is quite similar to the previous one in that it is a binary star system, a two-star multiple star system just like Tatooine (which by the way has become a new scientific term describing planets in multiple star systems after the hypothetical HD 188753 Ab, which could have been the first of the “Tatooine planets” was hypothesized back in 2005 but was later disproved) from Star Wars, but this time it has five medium-size “Neptune-mass” planets orbiting around the larger star 55 Cancri A, in order of distance from parent star: 55 Cancri e, b, c, f, and d (or 55 Cancri Ae, Ab, Ac, Af, and Ad to distinguish them from the other star, 55 Cancri B). This is the most number of confirmed planets orbiting one star other than the sun and hence raises the possibility of finding more planets around stars with or without known planetary systems by showing that our solar system of many planets is not unique.
2. Exoplanet’s Atmospheric Composition Known
HD 209458 b (November 5, 1999)
Another “special” gas giant in the sense that its orbital plane aligns perfectly with our line of sight i.e. it regularly passes through the face of its parent star as seen from Earth. This “transiting” of the planet allows us to more accurately calculate its size by analyzing the amount of its parent star’s light that is blocked off as it passes and more importantly determine the composition of its atmosphere through Spectroscopy, the study of interactions between radiation and matter (in this case, the interaction between gases and vapors in the planet’s atmosphere and its sun’s starlight). Using this method they have detected the presence of sodium vapor and more recently (October 2009) some water vapor, carbon dioxide, and methane in the planet’s atmosphere. It is also only the second planet revealed to possess organic compounds, with HD 189733 b being the first one on October 5, 2005.
1. Gas Giant in “Habitable Zone”
Gliese 876 b (June 23, 1998)
The habitable zone is the imaginary spherical shell surrounding a star where conditions are optimal for liquid water to exist on an Earth-sized planet orbiting within that shell. This gas giant is special because it orbits inside its sun’s habitable zone. “But so what,” you say, “how is it ‘habitable,’ we can’t live on gas giants, you know!” While it is true that we humans are as of now incapable of living on or in a gas giant, take a quick look at the gas giants in our solar system: they all have some relatively big, icy moons, and it is not impossible (albeit not certain) that Gliese 876 b could have some habitable moons (think Pandora and Polyphemus from the movie Avatar). Even if not, there is no reason not to believe that life could originate in or on gas giants, as evidenced by a paper from respected astronomers discussing the possibility of life in Jupiter’s thick atmosphere.
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