Top 10 Exoplanets: Weird Worlds in a Galaxy Not So Far Away [Slide Show]
A look at some of our extreme planetary neighbors right here in the Milky Way Galaxy
![Top 10 Exoplanets: Weird Worlds in a Galaxy Not So Far Away [Slide Show]](https://www.scientificamerican.com/sciam/cache/file/9880B98A-3B83-4D8E-9133CB6A5F7EEAB3.jpg?w=590&h=393)
Top 10 Exoplanets: Weird Worlds in a Galaxy Not So Far Away [Slide Show]
- BONUS: FIRST PROTOPLANET?: Though no exoplanets have been confirmed around a star called UX Tauri A, scientists have spotted a huge gap in its protoplanetary disk indicating that a planet may be coalescing. These swirling disks of gas and dust (leftover material from star formation) form around a star's equator and then extend outward into space. It is thought that bits of matter in these disks smash into each other and glob together, growing big enough in time to exert a strong enough gravitational force to suck in other particles. This snowball effect eventually yields planets that clear out the debris in their orbital paths. (The asteroid belt between Jupiter and Mars is an example of a section that never cleared fully that was only stabilized by the combined tugging of the sun and Jupiter.) Researchers believe Earth formed some four billion yeas ago when the sun was only 600 million years old. UX Tauri A has only been around for a million years, making its possible planet a contender for the youngest ever found. Star/Constellation Name: UX Tauri A/Taurus
Distance from Earth: 450 light-years
Distance from Host Star: between 0.2 and 56 AU (about the distance between Mercury and Pluto)
Year Discovered: 2007 NASA/JPL-Caltech - 1. PRIMORDIAL PLANET: Exoplanet PSR B1620-26 b is believed to have formed an incredible 13 billion years ago, less than a billion years after the big bang. Aptly nicknamed Methuselah, this probable gas giant resides in an ancient type of galaxy known as a globular cluster, where it orbits two stellar hosts, a white dwarf star and a pulsar, both remnants of larger stars. It is thought that Methuselah once orbited a common yellow star like our sun, which became a red giant, giving up its matter to a dense neutron star--the latter of which became a spinning pulsar in the process. Packed in amongst other stars in the cluster, scientists think it is likely that Methuselah has been blasted by radiation from many supernova explosions over its lifetime. Given the planet's violent history, it's unlikely that life could have arisen there, much less a Republic or an Empire. It, however, indicates that a long time ago, in a globular cluster far, far away, a world can exist. Exoplanet Name: PSR B1620-26 b
Star/Constellation Name: PSR B1620-26/Scorpius
Distance from Earth: 5,600 light-years
Exoplanet Mass: 2.5 Jupiters
Distance from Host Star: 23 AU (as far as the orbit of Uranus)
Year Discovered: 1994 NASA/G Bacon - 2. EARTH TIMES THREE: The exoplanet MOA-192 b, which orbits a purplish star in this artist's impression, is the smallest discovered so far, measuring about 3.3 Earths in mass. It revolves about a dim star that is about one twentieth the mass of our sun, making this the planet with the teensiest host star, to boot. This star's diminutive size, however, is quite common in the universe, so finding that it can sport planetary bodies encourages researchers about the odds of finding Earthlike planets. MOA-192 b was detected using gravitational microlensing--a phenomenon predicted by Albert Einstein that takes advantage of the light-magnifying effect of a star between an Earth observer and the object of interest, such as a suspected exoplanet. Exoplanet Name: MOA-192 b
Star/Constellation Name: MOA-2007-BLG-192-L/Sagittarius
Distance from Earth: 1,000 light years
Exoplanet Mass: 3.3 Earths
Distance from Host Star: 0.62 AU (closer than Venus, whose average orbit is 0.72 AU)
Year Discovered: 2008 NASA's Exoplanet Exploration Program - 3. IT'S A SMALL(ER) WORLD: Besides being the first exoplanet ever directly observed from Earth as it transited in front of its host star, exoplanet HD 209458 b (aka Osiris) is also shrinking. Its proximity to the inferno of its host star superheats the planet to an estimated 18,000 degrees Fahrenheit (10,000 degrees Celsius), which researchers believe is causing it to sweat off about 10,000 tons (9,000 metric tons) of atmospheric hydrogen every second, forming a cometlike tail. It is thought that HD 209458 b might eventually lose its entire atmosphere and just be left as a molten core of magma. The world was also the first exoplanet to give up evidence of water vapor in its atmosphere, followed by the discovery of methane. Exoplanet Name: HD 209458 b
Star/Constellation Name: HD 209548/Pegasus
Distance from Earth: 153 light-years
Exoplanet Mass: 0.69 Jupiter (though it is a third bigger in diameter than Jupiter with atmospheric swelling)
Distance from Host Star: 0.045 AU
Year Discovered: 1999 ESA/Hubble - 4. EXOHOTTIE: HD 149026 b ranks as one of the hottest known exoplanets, with a lead-boiling surface temperature of around 3,700 degrees Fahrenheit (2,000 degrees Celsius). Tricky measurements of light reflecting from its surface suggest that this world may be pitch-black in color, perhaps because of a strangely high concentration of heavy, metallic elements in its atmosphere. But even in that case, it may glow red like an ember from all that heat. Besides its fearsome exterior, researchers believe this gaseous "hot Saturn" has the largest known planetary core, estimated at about 70 to 90 Earth masses. Exoplanet Name: HD 149026 b
Star/Constellation Name: HD 149026/Hercules
Distance from Earth: 256 light-years
Exoplanet Mass: 0.38 Jupiter
Distance from Host Star: 0.042 AU
Year Discovered: 2005 NASA/JPL-Caltech; N2K Consortium - 5. NOT TOO HOT OR TOO COLD?: When astronomers spotted Gliese 581 c, it set off a flurry of reports that this exoplanet fell within the so-called "Goldilocks" zone where liquid water can exist on the planet's surface. Gliese 581 c orbits closer to its star than torrid Mercury orbits the sun, but the host is a red dwarf star 50 times cooler than our sun, which researchers thought placed it in that star's habitable zone. But further studies revealed that Gliese 581 c is probably slightly too close its star. A neighboring planet, Gliese 581 d, also skirts the estimated boundary of the zone but appears to be a bit too far out, receiving a similar amount of energy from its star as Mars does from the sun. Still, scientists think "d" may be the better candidate for life. And given that Gliese 581 is the 87th closest star to Earth, that's certainly not a bad place to start looking harder. Exoplanet Name: Gliese 581 c
Star/Constellation Name: Gliese 581/Libra
Distance from Earth: 20.5 light-years
Exoplanet Mass: five Earths (0.016 Jupiters)
Distance from Host Star: 0.073 AU
Year Discovered: 2007 ESO - 6. FREAKISHLY FROZEN WORLD: Scientists think Gliese 436 b (aka GJ 436 b), a Neptune-size exoplanet, is too heavy to be all gas but not heavy enough to be entirely rock. They surmise that in addition to gas and rock, it also contains a kind of pressurized, high-temperature ice that only exists on Earth in laboratories, where it goes by names like "ice VII" and "ice X." The high pressures deep inside the planet may stabilize this alternate solid state of water, similar to the way intense pressures in Earth's crust can squeeze carbon atoms into crystalline diamond. In this artist's impression, the probable hydrogen and helium in Gliese 436 b's atmosphere gives the exoplanet an azure complexion. Exoplanet Name: Gliese 436 b, or GJ 436 b
Star/Constellation Name: Gliese 436 or GJ 436/Leo
Distance from Earth: 33 light-years
Exoplanet Mass: 22 Earths (0.07 Jupiter)
Distance from Host Star: .029 AU
Year Discovered: 2004 Lynette Cook, courtesy of Geoff Marcy - 7. POTPOURRI OF PLANETS: Astronomers discovered a fifth planet around the sunlike star 55 Cancri in 2007, lending a familiar feel to this solar system and making it the most planet-populated one outside our own--so far. All five confirmed planets in the system are jumbo versions of Earth and its neighbors, including a rocky mega-Earth and a gas giant four times as massive as Jupiter. Recent work predicts another two celestial bodies on 55 Cancri's planetary merry-go-round. And to make matters even more complicated, a small red dwarf star also orbits the primary star at about 1,000 AU. Overall, the crowded 55 Cancri house shows that multiplanet arrangements may in fact be relatively common. Exoplanet Names: 55 Cancri b through f
Star/Constellation Names: 55 Cancri/Cancer
Distance from Earth: 44 light-years
Exoplanet Masses: Range from 18 Earths to four Jupiters
Distance from Primary Host Star: between 0.04 and 5.9 AU
Year Most Recent Exoplanet Discovered: 2007 NASA/JPL-Caltech; JPL - 8. SURVIVOR OF APOCALYPSE: V391 Pegasi b distinguishes itself as the only planet known to orbit a star that has passed through its red giant phase. As stars like our sun age, they run low on nuclear fuel in their cores and swell to hundreds of times their original sizes. When our own sun goes red giant in about five billion years, it will likely swallow Mercury and Venus and, if it doesn't also envelop Earth, will scorch the planet: boiling off Earth's oceans, eventually leaving the once-verdant world a barren cinder. Scientists think V391 Pegasi b started out about the same distance from its star as Earth. This gap then shrunk as the star expanded to about 0.3 AU, but the exoplanet has since migrated out farther than Mars's orbit as its star shed mass. V391 Pegasi b's durability may bode well for the fate of Earth and other planets orbiting future red giants—albeit not for any inhabitants. Exoplanet Name: V391 Pegasi b, or V391 Peg b
Star/Constellation Name: V391 Pegasi/Pegasus
Distance from Earth: 4,550 light-years
Exoplanet Mass: 3.2 Jupiters
Distance from Host Star: 1.7 AU
Year Discovered: 2007 Mark Garlick © HELAS - 9. TYPICAL STAR; EXTRAORDINARY PLANET: The first exoplanet spotted around a typical or "main sequence" star similar to our sun, gaseous 51 Pegasi b completes an orbit around its host star every four days. Many exoplanets found after this one are very similar "hot Jupiters," named for their size and proximity to their star. Such worlds turn up frequently in a common technique for hunting exoplanets called the radial velocity method. With it, scientists look for regular changes in a star's emitted light caused by the tugging of an exoplanet. 51 Pegasi b is nicknamed Bellerophon, after the mythical Greek hero who captured the winged horse Pegasus. Exoplanet Name: 51 Pegasi b
Star/Constellation Name: 51 Pegasi/Pegasus
Distance from Earth: 48 light-years
Exoplanet Mass: 0.47 Jupiter
Distance from Host Star: 0.05 AU
Year Discovered: 1995 Debivort - 10. FIRST EXOWORLD: The first solid evidence for an exoplanet (extrasolar planet) came in 1992 when scientists calculated that two bodies must be orbiting the pulsar PSR 1257. Pulsars are rapidly spinning neutron stars (the superdense remnants of massive stars that have exploded as supernovae), which emit radio waves in pulses. Researchers think the two planets are the leftover rocky cores of gas giants whose atmospheres were completely blown away in the prior supernova event or they may have formed in a second round of planet formation after the supernova. Exoplanet Names: PSR 1257 b and PSR 1257 c
Star/Constellation Name: PSR 1257/Virgo
Distance from Earth: 978 light-years (One light-year equals the distance light in a vacuum travels in a year: 5.88 trillion miles, or 9.46 trillion kilometers.)
Exoplanet Mass: 4.1, 3.8 Earths (0.013 and 0.012 Jupiter)
Distance from Host Star: 0.36, 0.46 AU (An AU, or astronomical unit, is the average distance between Earth and the sun, which is about 93 million miles, or 150.6 million kilometers.)
Year Discovered: 1991 NASA/JPL-Caltech
