These are exoplanets with intriguing or unique characteristics. The list will evolve and expand over time so long as our quest for knowlege on this new frontier remains strong and robust. Intriguing extrasolar planets as imagined and portrayed by other may be found at these sites from space.com and Scientific American.

Closest Exoplanet to Earth
Most Planets Orbiting Star
Most Eccentric (eliptical) Planetary Orbit
Largest Planet
First Directly Observed Exoplanet
Directly Observed Exoplanet Closest to its Star
First Detection of Organic Molecule and Water
Is it a planet or a star?

Closest Exoplanet to Earth

As of early 2008, Epsilon Eridani is the closest star to earth (10.5 light years) where an exoplanet has been discovered. It is a main-sequence K-2 star that is smaller, redder, and dimmer than the sun. It is much younger than the sun, being only 800 million to a billion years old.

Epsilon Eridani has a cold dust disk located at about where the Kuiper Belt of the Solar System would be -- from inside Neptune's orbit to twice Pluto's average distance from the Sun.

The confirmed planet of this star, Epsilon Eridani b, is a 2500 day-period Jupiter-like (mass = 1.55 Jupiter) planet that orbits at an average 3.3 AU from the star.

This is one of the most eccentric orbits of any extrasolar planets — 0.7. For comparison, an orbit in a perfect circle would have a zero eccentricity rating. At its closest point, the planet would be 1.01 AU from the star, at the furthest it would be 5.77 AU. Given this eccentricity of orbit and a heat output from the star less than that of the sun, this planet is outside the habitable zone.

Most Planets Orbiting Star

Artist's concept of the 55 Cancri system with the habitable zone shown in green

Astronomers toward the end of 2007 discovered an unprecedented fifth planet orbiting the sunlike star 55 Cancri, 41 light-years away. The star is slightly cooler than our sun. An excellent graphical presentation on the planet and its discovery can be found at this NASA site. This quicktime video animates the planetary system.

The five planets orbit along relatively circular paths (with an eccentricity rating of between 0.014 and 0.2).

The newest discovery, 55 Cancri f, has the most eliptical orbit. It is about half the mass of Saturn and is fourth out from the star at 0.738 AU in a large gap between the third and fifth planets. It orbits 55 Cancri in 260 days. The location, similar to Venus in our system, places it in the estimated habitable zone of this star slightly cooler than our sun.

55 Cancri d, the most distant planet from the star is the size of four Jupiters. It orbits at roughly the same distance that separates Jupiter from the sun.

A new competitor for 55 Cancri is HD 10181, a sun-like star at a distance of 127 light-years from Earth. In 2010, a European group using the HARPS spectrometer at the 3.6 meter telescope in La Silla, Chile, detected 6 planets orbiting the star with the possible presence of at least one more planet. The six are at about the size of Neptune, being between 13 and 25 Earth masses. These planets circle their star in orbits that range from six to 2200 days at distances between 0.06 and 3.4 AU. The third largest planet has an eliptical orbit of about 600 days, equivalent to a distance just beyond Earth to outside Mars' orbit. If confirmed, the 7-planet system would be nearly as large and complex as our own 8-planet solar system.

Most Eccentric (eliptical) Planetary Orbit

HD 80606 b is an extrasolar planet orbiting the star HD 80606 at an average distance of 0.439 AU. Based on its mass, 3.4 times that of Jupiter, it is likely to be a gas giant. It orbits the star in about 111 days.

HD 80606 is a star of the G5 type slightly smaller than the sun. It is part of a binary star system of nearly identical yellow main sequence stars about 190 light years away. The two stars are greatly separated by a distance of 1200 AU.

As of early 2008, HD 80606 b has the most eccentric orbit of any known extrasolar planet. It has an eccentricity of 0.934, comparable to that of a comet in our solar system. The eccentricity may be a result of the Kozai mechanism, which would occur if the planet's orbit is significantly inclined to that of the binary stars.

As a result of this high eccentricity, the planet's distance from its star varies from 0.03 to 0.88 AU. At the far point it would receive an insolation similar to that of Earth, while at closest to the star the insolation would be far greater than that experienced by Mercury in our solar system. This would likely result in extreme seasonal variations. Needless to say, neither the planet nor any associated moons would be habitable.

Researchers used NASA's Spitzer Space Telescope obtained infrared measurements of the heat emanating from the planet as it whipped behind and close to its star. In just six hours, the planet's temperature rose from 800 to 1,500 Kelvin (980 to 2,240 degrees Fahrenheit). Astronomers at the University of California, Santa Cruz have generated realistic images of the planet by feeding the data relating to the heating of the planet into computer simulations of the planet's atmosphere.

Largest Planet

GQ Lupi is a young T-Tauri star (a star still in the formation process), located in a region of star formation about 400 or 500 light-years away. The star GQ Lupi is apparently a very young object still surrounded by a disc, with an age between 100,000 and 2 million years. In 2004, a companion was discovered separated from the star by about 100 AU (or 100 times the distance between the Sun and the Earth). This is roughly 2.5 times the distance between Pluto and the Sun.

The companion, GQ Lup b is potentially the largest exoplanet discovered. However, it is unclear if it is an actual exoplanet or a brown dwarf, a "failed" star not massive enough to centrally produce major nuclear reactions. Although the borderline between the two is still a matter of debate, one way to distinguish between the two is by their mass. Giant planets are lighter than about 13 Jupiter-masses (the critical mass needed to ignite deuterium fusion). Brown dwarfs are heavier.

The new observations do not provide a direct estimate of the mass of GQ Lupi b. The problem is that for such very young objects, traditional theoretical models are probably not applicable. If they are used, however, they provide an estimate of the mass of the object that lies somewhere between 3 to 42 Jupiter-masses, i.e. encompassing both the planet and the brown dwarf domains.

The largest exoplanet that is likely less than 13 Jupiter masses, and therefore not a brown dwarf star, is XO-3 b. XO-3 b has an orbit around its parent star of less than four days. The orbit is significantly elliptical instead of circular, as would be expected at this distance. The radius of this object is 1.92 times that of Jupiter, which would make it the largest of any known extrasolar planets.

The planet is the most massive planet found in close proximity to a star. It is also a transiting planet, passing in front of its parent star during each orbit. It is the third such planet to be found by the XO Project which was specifically created to locate transiting planets.

First Directly Observed Exoplanet

Obtaining images of an exoplanet would enable scientists to study in detail the physical nature of the object and, in particular, to analyse the composition of its atmosphere. The ultimate goal is to perform such analysis for earth-sized planets, in the hope of detecting a telltale signature of extraterrestrial life.

Infrared image of 2M1207 (blueish) and 2M1207b (reddish). The two objects are separated by less than one arc second in Earth's sky.

That objective is still decades in the future, as earth-size and even Jupiter-size planets around stars as old as the Sun are too faint to be detected by present-day technology.

Nevertheless, great progress can be achieved by taking images of giant planets orbiting much younger objects. Because giant planets a few tens of millions of years old are much hotter and brighter than their older brethren, they can be much more easily detected.

2M1207b is an extrasolar planet orbiting the brown dwarf 2M1207 about 170 light years from Earth. The brown dwarf 2M1207 has approximately 25 times the mass of Jupiter and is thus about 42 times lighter than the Sun. As a member of the TW Hydrae Association, it is about eight million years old.

2M1207b was one of the first candidate extrasolar planets to be directly observed (by infrared imaging). It was discovered in 2004 by the Very Large Telescope at the Paranal Observatory in Chile by a team from European Southern Observatory. Later observations unambiguously established that this was the first direct image of a planetary-mass companion in orbit around a self-luminous body, other than our Sun.

The gas giant planet is very hot (1600 K), mostly due to gravitational contraction. The spectrum of the planet presents the strong signature of water molecules. Based on the infrared colours and the spectral data, evolutionary model calculations lead to the conclusion that 2M1207b is a 5 Jupiter-masses planet. Its mass can be estimated also by use of a different method of analysis which focuses on the strength of its gravitational field; this technique suggests that the mass might be even less than 5 Jupiter mass, well below the calculated limit for deuterium fusion in brown dwarfs. The distance between 2M1207b and its star is around 46 AU (similar to the mean distance between Pluto and the Sun), giving it an estimated orbital period of around 1700 years.

An interesting twist in this story occured early in 2008. The puzzling high temperature stimulated some scientists to reevaluate the data. They developed a new theory that 2M1207b was the result of a recent collision between two protoplanets, heating the now expanded planet. The theory will be tested.

Directly Observed Exoplanet Closest to its Star

The European Southern Observatory (ESO) confirmed through direct imagery a planet orbiting in the plane of the debris disk of the star Beta Pictoris. The star is located 63.4 light years from our solar system. It is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris system is very young, only 8–20 million years old.

As of 2009, this planet was the closest extrasolar planet to its star ever photographed. At about 8 AU, it is roughly equivalent to the distance between Saturn and the Sun.

The composite image represents the close environment of Beta Pictoris as seen in near infrared light. This view is revealed after a very careful subtraction of the much brighter stellar halo. The outer part of the image shows the reflected light on the debris disc. The inner part is the innermost part of the system, as seen on the Very Large Telescope. The newly detected source is more than 1000 times fainter than Beta Pictoris.

First Detection of Organic Molecule and Water

This is one weird planet. HD 189733 b is a hot gas giant planet that closely orbits around the yellow dwarf star HD 189733 A. The planet, 63 light-years away, was discovered in 2005 when astronomers observed it transiting across the face of the star. The mass of the planet is estimated to be 15% larger than Jupiter's. The planet completes an orbit every 2.2 days. HD 189733b’s atmosphere swelters at 900 degrees C, about the same temperature as the melting point of silver.

The planet represents a first on the road to discovery of life outside our Solar System. The NASA/ESA Hubble Space Telescope in 2007 detected an organic molecule, methane, in the atmosphere of this planet and confirmed the presence of water. Although methane has been detected on most of the planets in our Solar System, this is the first time any organic molecule has been detected on a world orbiting another star.

The observations using spectroscopy, which splits light into its components to reveal the “fingerprints” of various chemicals, were made as the planet HD 189733b passed in front of its parent star in what astronomers call a transit. As the light from the star passed briefly through the atmosphere along the edge of the planet, the gases in the atmosphere imprinted their unique signatures on the starlight from the star HD 189733.

Is it a planet or a star?

A 2010 discovery in an odd star system about 450 light-years away introduces a controversy over the upper limit for planet size. The system includes an object that is well within the usual mass range for a planet - five to 10 times as massive as Jupiter. But here's the problem: The location and age of the object suggest that it had to congeal from its own cloud of gas and dust, like a star rather than a planet.