| Uranus |
|
The seventh major planet of the solar
system in order from the Sun, discovered by William Herschel in 1781. It is just
bright enough to be seen by the naked eye under good observing conditions. From the Earth, it appears as an almost
featureless greenish disc, in even the largest telescope. In 1986,
the space probe Voyager 2 passed close to Uranus and its satellites, providing
close-up images of them. Ten small satellites were discovered by Voyager 2. Five larger satellites were already
known: Miranda,
Ariel,
Umbriel,
Titania and
Oberon, and two more were discovered in 1997. Uranus is one of the four "gas giant" planets of the solar system, with a diameter four times the Earth's and a mass fifteen times greater. It is composed almost entirely of hydrogen and helium. It is generally believed that there is a small rocky core at the centre of the planet, which is surrounded by a thick icy mantle of frozen water, methane and ammonia which merges into the outermost layer, an atmosphere of hydrogen and helium, with small quantities of some molecular compounds. Even in the Voyager close-ups, Uranus presents a bland, nearly featureless appearance, though there is some evidence for faint bands parallel to the equator. A curious feature of Uranus is that its rotation axis lies almost in the plane of the solar system, rather than being nearly perpendicular to it, as is the case for the other planets. The internal rotation period is 17 hours 14 minutes. In 1977, a series of narrow rings was discovered around Uranus in its equatorial plane. The rings are each only a few kilometres wide and not visible from Earth. The discovery occurred when Uranus occulted an eighth magnitude star. The rings caused small dips in the observed brightness of the star just before and just after the occultation by the disc of the planet. Later occultations, of Beta (b) Scorpii and Sigma (s) Sagittarii, confirmed the result. The ring system was subsequently imaged by Voyager 2 in 1986, when two further rings were discovered, bringing the total to eleven. |