Most of the planets discovered orbiting other stars, are found to be gas giants orbiting close to their star. In many cases they receive much heat and are therefor called hot jupiters.
This does not mean that all gas giants orbit close to their stars. Rather the result reflects the studies conducted on the stars in the sense that they have occurred in short intervals. So planets that orbit their stars fast (which means that they are very close to the parent star)
have been the first to be discovered. Undoubtedly there are gas giants further away in cooler regions, but their presence could take decades to detect. To determine the orbital period and mass of a planet, astronomers need to follow it for atleast one complete orbit.
Below you can read about some gas giants found till this date.
51 Pegasi B - Bellerophon
51 Pegasi B, or Bellerophon as the gas giant has been dubbed, was the first extrasolar planet to be discovered around a sunlike star. It was found by a Swiss team in 1995, lead by Didier Queloz. Jeff Marcy and Paul Butler (the Swiss team's rivals) later confirmed their discovery to be authentic. The parent star is found at a distance of about 48 light years.
Didier Queloz's first reaction was that there must be something wrong with the data since the planet (if it really was a planet) would have been very different from what we expected it to be. For instance, it was found extremely close to its' parent star (7.5 million km or 0.05 AU), which contradicted astronomers' theory of how solarsystems are created.
Astronomers today explain the closeness of the planet by suggesting that the planet formed further away and gradually migrated towards the star. The migration was caused by dust in its orbit, gradually slowed down the orbit and hence forced the planet closer. During this process it probably bulldozed smaller planetesimals orbiting within the orbit of Bellerophon.
There is also a possibility that the planet may have thrown them away from the solar system.
Presently, the planet orbits very close to its' star (one orbit, or a Bellerophonian year is about 4.2 Earth days long), where the temperature might be as high as 1 300 degrees Kelvin (or even as high as 2000-3000 K). It is possible that the extreme heat would blow away gas from the planet, giving rise to a structure which reminds of a cometary tail.
Though calculations show that the loss of gas from the atmosphere may only be 5% during the entire lifespan of the star!
The metal nickel which is present in its atmosphere has evaporated due to the heat and may be giving the gas giant's atmosphere a bluish tint.
Though the planet is only about half the mass of Jupiter, its' radius could be 30% larger (approx. 93 000km). The heat from the star makes the gas giant to expand, hence the radius.
It has been calculated that the planet may be tidally locked to its' star, which makes it always show the same face towards the sun. For this reason, the planet may not have the oblate shape that the four jovian planets in our solarsystem have. Though powerful winds that circulate the planet in less than 4.2 days would appear to falsely contradict this.
Back to Top.
47 Ursae Majoris
This system is located about 46 light years from Earth. On the night sky it appears in the constellation of Ursa Major, or The Big Bear. It is the solarsystem which shows most resemblance to our own. The host star is calculated to be 1.03 times more massive than our sun, 1.26 times larger in diameter and about 1.5 times brighter in luminosity.
Currently two gas giants have been found to orbit the star. They're called
47 Ursae Majoris b (47 UMa b for short) and 47 Ursae Majoris c (47 UMa c for short). The first gas giant orbits further away than Mars does in our solarsystem (315 million kilometers, 2.10 AU). One orbit around the star takes 3 years for it to complete. 47 UMa b has an estimated mass of 2.54 Jupiters and was discovered in 1996.
The second planet was announced in 2002 by Debra Fischer and Jeff Marcy. It has a mass of 0.74 Jupiters and orbits the star in 7.10 years at a distance of 3.73 AU (560 million km).
Both planets have a low eccentricity in their orbits, which makes the orbits stable. These kinds of orbits are appreciated by astronomers while searching for habitable planets.
There is a huge gap between the star and 47 UMa b, void of gas and dust. This gap could very well contain smaller planets. It has been calculated that a rocky planet orbiting between 1.05 and 1.83 AU from the star could have liquid water present on its' surface.
Unlike the other gas giants mentioned above, these gas giants receive much less energy from their parent star, which makes them cold.
Back to Top.
41 light years from Earth, in the constellation of Cancer, astronomers have discovered the only system of four planets orbiting a sunlike star. This system orbits around the star 55 Cancri, which actually is a binary star system. The main component is a sunlike star with about 95% mass compared to our sun, and about 60% of its' luminosity. The second component is a red dwarf
and orbits the main star at a distance of 1065 AU (1 AU = 150 million km). It takes this red dwarf thousands of years to complete an orbit around the main star.
In 1998 a disc was discovered orbiting the main star at a distance which ranges from 27 - 40 AU. It resembles the Kuiper Belt in our solarsystem.
In 2005, astronomers discovered what is now believed to be the innermost planet, named 55 Cancri e, using the radial velocity method. It has a mass of 10-20 Earth's (Neptune sized) and is a new class of planets, called Super Earth's. This kind of planets could very well be rocky and have thick atmospheres. 55 Cancri e orbits the star in less than 3 (2.8) days,
and the temperature at its' distance (about 5.6 million km.) is about 1 200 degrees Kelvin. The planet might have been much larger in the past, but the radiation from the star has stripped hydrogen and helium from the atmosphere, leaving only the core behind. Its possible that the crust is very thin on this planet and that it would be constantly resurfaced by volcanoes.
55 Cancri b was discovered in 1996 and was among the first extrasolar planets to be discovered. It orbits the main star at a distance of about 17 million km and the temperature may be around 700 K (this high temperature might have made the planet to swollen up to about 120% of Jupiter's radius), which is similar to that of Mercury. The mass of this planet is just a few percent less than that of Jupiter.
This planet takes about 14 days to complete an orbit.
55 Cancri c takes 44 days to complete an orbit, which is quite eccentric, ranging from 0.13 to 0.34 AU. This planet has a mass slightly higher than 1/5th of Jupiter.
55 Cancri d's orbit is similar to Jupiter's: It takes the planet about 5 AU away from the main star. Its mass is about 4 times greater than Jupiter's.
Back to Top.
70 Virginis - Goldilocks
Located in the constellation Virgo, the planet 70 Virginis b is a massive planet. It is atleast 6.6 times heavier than Jupiter (maybe as much as 8 times heavier). Some studies have indicated that the planet actually might be much more massive and thus be a brown dwarf, though it is today thought to be a planet indeed. 70 Virginis b orbits its' star in 117 days at a distance of 72 million km (0.48 AU). The temperature at this distance is about 480 degrees Kelvin, which allows water to exist as vapour in the atmosphere.
The star which it orbits is located 59 light years away and is a sunlike star, thought to be about 15% more massive and three times more luminous than our sun. 70 Virginis b is the second planet discovered (1996) to orbit a sunlike star, the first was 51 Pegasi b.
The planet is sometimes referred to as Goldilocks.
Due to the relatively low temperature at the orbit of Goldilocks, clouds are unlikely to form. The planet may be monotone is colour, perhaps blue (coloure by methane, like on Neptune?).
Because of the planets' massive gravity, it is quite possible that Goldilocks has very many moons, perhaps more than hundred (our own Jupiter has 63 known moons), of various sizes. Some might be larger than Earth, but water oceans are unlikely to exist. If moons are close enough, they might experience similar tidal forces that Jupiter's moon Io is facing. This would make the moons' crust to be atleast partly molten and scarred with volcanoes.
Moons located further away would probably be heavily cratered from asteroids that the planet has pulled towards it. The gravity of Goldilocks will easily capture, or alter the course of objects' passing too close.
Back to Top.
Scientists believe that the star Epsilon Eridani, located about 11 light years away, in the constellation of Eridanus (the river), hosts atleast one gas giant. There is evidence pointing to that the system is newly born in astronomic terms, perhaps only a few hundred million years. The system also hosts a belt of icy objects which reminds of the Kuiper belt.
Comets and asteroids probably hit the planets regularly, many of these objects might come from the belt of icy objects, which is thought to contain a mass of about 1.2 Jupiter masses, or about 1000 times more matter than is found today in our Solar System's own inner region. Astronomers might have much to learn from this system, because it resembles conditions thought to be present when our own solarsystem formed.
The discovery of the planet was announed in 2000. It is believed to orbit the star at 3.3 AU in a highly eccentric orbit, which takes it about 6.8 Earth years to complete. The planet is 1.3 AU from the star at periastron, and 5.3 AU at apastron. The planet has a mass of 0.8 - 1.6 Jupiters.
In 2002 a second planet was announced to orbit Epsilon Eridani. The new planet orbits the star in about 280 years and might have the double mass of Neptune.
Back to Top.
The planet around the star Tau Bootis has among the shortest orbiting periods known. It circles its's star in about 3.3 days at a distance of 0.046 AU, or 6.9 million kilometres. The mass is thought to be atleast 4 times that of Jupiter.
The parent star (F class star) is located 51 light years away in the constellation of Bootes. It has a mass of about 1.3 times the sun's mass, its' surface temperature is around 6 300 K, which makes it shine 3.1 times brighter than a sun.
The radius of the star is estimated to be 1.46 times greater (roughly 1 020 million km).
The primary star has a red dwarf companion which orbits it at an average distance of 245 AU (37 billion km). The orbit takes about 2 000 years to complete.
Needless to say, the combination of the bright primary star and the very tight orbit makes the planet Tau Bootis b very hot. The temperature could be as much as 1 600 K, which would make it the hottest planet discovered.
The planet may be so hot that the gas in the atmosphere is driven away by the heat and solar winds, creating a similar faint and comet-like tail that also might be found around the planet 51 Pegasi (which is named Bellephoron). If it is true that the planet is loosing its' atmosphere to space, the planet might have been larger in the past.
The planet would appear a bit blurry and the clouds of silicates would be obscured by this blurryness.
It is known that a star is likely to tidally lock the planet so that the planet always shows the same "face" towards the sun, but it is a great surprise when astronomers discover that a planet is tidally locking its parent star, which is many times massive than the planet. Nevertheless, recent studies suggest that this is the case on Tau Bootis.
As mentioned above, the planet orbits the main star in about 3.3 days, but the main star also spins once around its' axis in 3.3 days. This fast orbital period is very fast for a star of Tau Bootis' class. The tidally locking might be contrained to the outer layers of the star, just like the moons tidally locking only affects the oceans while the crust remains unaltered by the moon.
Back to Top.
The star Upsilon Andromedae is an F (F8) class star. It is found in the constellation of Andromeda, 44 light years away. Its' surface temperature is 6210 K and the luminosity is 3.4 higher and the mass is 1.3 times more than our sun. Upsilon Andromedae was the first star announced that harbors a system of planets, not just one planet. The announcement was made in 1999.
All the three planets discovered have a mass similar to Jupiter. The first planet, Upsilon Andromedae b, orbits at a distance of 0.059 AU, in 4.6 days. It has a mass of 0.69 Jupiters. The temperature at this distance would be about 1 400 K, which makes it a "hot jupiter".
The second planet (Upsilon Andromedae c) orbits just nearby, at a distance of 0.83 AU from the star. It is much cooler than the innermost known planet: the average temperature is 380 K. This planet has a mass that is almost the double (1.89) of Jupiter.
The third planet, Upsilon Andromedae d is the most massive (3.75 Jupiters) of the planets discovered. The orbit takes it 2.53 AU (about 380 mill. km.) on average. At this distance the average temperature is low: 217 K. Due to its' mass, it is likely that this planet has many moons, some that are very large. It is likely that some moons have icy surfaces, just like the jovian moons of Jupiter.
Back to Top.
Previous: Carbon Planets.
Next: Earth-Sized Planets.
51 Pegasi B - Bellerophon
- Space Art (P) - Space Art 2
- Hot Jupiter (P)
- Gas Giant (P)
47 Ursae Majoris - Space Art 3
70 Virginis - Space Art 4
Epsilon Eridani - Space Art 5
1 Jupiter Mass = 1.9 x 10^27 kg,
1 AU = 150 million km.
Space art: A of a Hot Jupiter, a Gas Giant, from the surface of the star.
This illustration is available upon request, as a print (6000x3000 pixels, 300 dpi).
Space art: A Gas Giant, or, a "Hot Jupiter". These planets are found extremely close to their host stars. The heat from the star blows off the atmosphere of the gas giant into space.
This illustration is available upon request, as a print (8000x6000 pixels, 300 dpi).
Space art: An Extrasolar Gas Giant.
This illustration is available upon request, as a print (4800x6000 pixels, 300 dpi).
Above: 51 Pegasi b, Bellerophon. This planet is so close to its' star that the temperature is about 1 300 degrees Kelvin. Metals, like lead have evaporated, which in turn gives the planet's atmosphere a bluish tint.
The intense heat might have formed a faint comet-like tail pointing at the opposite direction of the star, which can be seen on this painting.
Above: This is the extrasolar planet 47 Ursae Majoris c, the second planet of the system. It is less massive than Jupiter: cirka 0.71 Jupiter masses.
Above: 70 Virginis B - Goldilocks is a huge planet. It is thought to have atleast 6.6 times the mass of Jupiter. The planet may have many more moons orbiting it than Jupiter (63). Some of them may be the size of Mars or Earth.
Above: Present in a solarsystem which is thought to be newly born, Epsilon Eridani b might be heavily bombed with comets and asteroids. Epsilon Eridani b orbits its' star at an average distance of 3.3 AU. Though its' orbit is highly eccentric.