Voyager 1, now in interstellar space, sent back a final present a few years ago: a “family portrait” of the solar system, showing a shrunken sun and several tiny flecks of light–the planets. One of those flecks, a tiny blue dot, is the Earth.
The past 20-some years has seen an explosion in planetary knowledge as robot spacecraft have toured all the planets but Pluto and sent back some really neat postcards.
Let’s take a walk around the neighbourhood; I think you’ll agree we’ve got the nicest house.
Take Mercury, for example. It’s a nice, sunny place, being only about one third as far away from the sun as we are: the average daytime temperature is 330 degrees Celsius. (But at least it’s a dry heat!) It cools right down at night, though; right down to -180. Good sleeping weather!
Mercury, photographed by Mariner 10 in 1974, looks like our moon, with similar craters, lava flows, and dust-covered hills and plains. It orbits the sun in only 88 Earth days, and rotates on its axis every 59 Earth days. (For many years it was thought Mercury always kept the same face to the sun–the “hot” and “cold” sides of Mercury featured in a lot of science fiction stories.) The planet’s diameter is 4,880 kilometers, it has no satellites, and it has virtually no atmosphere.
Venus, on the other hand, has lots of atmosphere: so much atmosphere you can’t see the surface because of the clouds. This made it easy to imagine huge oceans and jungles on Venus, until we got a spacecraft there to see for ourselves.
Alas, Venus’s atmosphere is mostly carbon dioxide–the infamous “greenhouse gas.” The rocky surface, revealed by the Russian landers Venera 13 and 14, as well as by radar scans, has a toasty mean temperature of about 480 degrees. If the planet ever had oceans, they’ve long since evaporated. The atmospheric pressure is 90 times that of Earth, and the clouds are composed of sulfuric acid.
Venus, has a diameter of 12,100 kilometres, orbits in 224.7 days, and rotates in 243 days. It has no satellites.
Earth we’re all pretty familiar with. Suffice it to say, its mean distance from the sun is 149.6 million kilometres, its year is 365 days, its day is 23 hours and 56 minutes, and its diameter is 12,756 kilometres. Its atmosphere is made up mostly of nitrogen, oxygen and car exhaust.
Mars was populated by little green men who dug canals until the Mariner orbiters and Viking landers actually went there. It still fascinates us, though, and it will be the first planet humans visit in person, probably within 20 years.
Mars, like Venus, has a carbon-dioxide atmosphere, but it’s very thin–atmospheric pressure is about 1/100 of Earth’s. The temperature ranges from about 27 degrees at the equator to a minimum of about -138 over the winter pole, giving a mean of about -40.
Tourist attractions on Mars include Olympus Mons, a volcano 25 kilometres tall; the Vals Marineris canyon, six kilometres deep; polar ice caps, and giant, planet-wide dust storms.
At a mean distance of 227.9 million kilometres from the sun, Mars orbits in 687 Earth days, has a day of 24 hours, 37 minutes, is 6,794 kilometres in diameter, and boasts two satellites, Phobos and Deimos.
Next comes giant Jupiter, largest planet in the solar system. Composed of hydrogen and helium, it has no solid surface, and for some reason it radiates twice as much heat into space as it receives from the Sun. It has a faint ring, made of microscopic rocky particles, at least 16 satellites (many of them very interesting in their own right), and is 143,200 kilometres in diameter. It orbits the sun (at a mean distance of 778.3 million kilometres) every 11.86 years, and rotates in nine hours, 55 minutes. Its best-known feature is the Great Red Spot, a huge storm bigger than Earth.
Saturn, of course, is known for its rings, which, unlike Jupiter’s, can be seen from Earth. They’re made up of particles ranging from tiny specks of dust to icy chunks many metres in diameter. Like Jupiter, Saturn is a gas giant–mostly hydrogen and helium–that radiates more heat than it receives. It has at least 21 satellites, its mean distance from the sun is 1.427 billion kilometres, it orbits once every 29.46 Earth years, and its rotation period is 10 hours and 40 minutes. It’s a bit smaller than Jupiter, with a diameter of 120,000 kilometres.
Uranus also has rings, at least 11, made up of large rocky chunks, and at least 15 satellites, including 10 discovered by Voyager 2. Its atmosphere contains helium, hydrogen and methane, and its effective temperature is only about -213 degrees. Its mean distance from the sun is 2.87 billion kilometers, it orbits in 84 years, its “day” is 17.2 hours, and it’s 51,800 kilometres in diameter.
Neptune has a blue atmosphere with storm systems similar to Jupiter’s, including a large storm called the Great Dark Spot. It also has two narrow rings and two broader ones, and at least eight moons, six of which were discovered by Voyager 2. Its atmosphere, too, contains hydrogen, helium and methane, but its temperature is much higher than Uranus’s, because it has an internal heat source like Jupiter and Saturn–though nobody is sure what those heat sources are. At 4.497 billion kilometres from Earth, it orbits once every 165 years and rotates in 18 and a half hours. It’s approximately 49,500 kilometres in diameter.
Finally, there’s Pluto, whose orbit is so eccentric that although its mean distance from the sun, 5.9 billion kilometres, is the greatest, for the last 20 years it has been closer to the sun than Neptune (it just slipped outside Neptune’s orbit again). Pluto orbits once every 248 years and rotates in six days, nine hours and 18 minutes. It’s approximately 3,000 kilometres in diameter and has a satellite, Charon, one third as large as itself–the largest satellite in proportion to its planet in the solar system. Both Pluto and Charon are apparently made of frozen water, ammonia and methane.
So there you have it–a brief tour of the solar system. These are our next-door neighbours, and if you think that Pluto is pretty far away to be next door, at 5,900,000,000 kilometres, just remember that the nearest star is more than 40,000,000,000,000 kilometres away.
On that scale, this is a very cozy neighbourhood indeed.
