As we look at the nighttime sky this time of year, we can see thousands of stars. At least two of them in the direction of Sagittarius and Capricornus are planets. The brightest one is also the 4th brightest object in the entire sky after the Sun, Moon and the nearby planet Venus. This is none other than Jupiter, the largest planet in the Solar System and the 5th planet out from our Sun.
While Jupiter has been known since ancient times, the first detailed observations of this planet were made by Galileo Galilei in 1610 with a small telescope.
Jupiter was originally formed along with the rest of the solar system about 4.5 billion years ago, when gravity pulled swirling gas and dust in to become this gas giant.
Jupiter is named after the Roman King of the Gods. It is slightly more than 5 times further from the Sun than Earth is. Jupiter also goes by the Greek Mythology name Zeus!
Jupiter’s diameter is 11 times bigger than Earth, about 86,881 miles. About a thousand Earths can fit inside its volume. The planet has no solid surface. Anyone trying to land on this planet will not succeed. You would just plummet deeper and deeper until the spacecraft you are in is crushed by the atmospheric pressures within its interior. Its atmosphere contains hydrogen, helium, methane and ammonia .
The most prominent feature one can see on Jupiter is its Great Red Spot, a cyclonic storm that can fit two Earths inside. This storm has been roaming in the same southern latitudes of Jupiter for over four hundred years. The storm packs winds up to 175 miles per hour.
Jupiter orbits the Sun every 12 years. What this means for amateur astronomers is that it covers one constellation of the zodiac each year. It has faint small rings as first viewed from Voyager 1 in 1979. Its rotation is the fastest of all the planets: only 9 hours and 55 minutes for Jupiter to make one single day!
Its cloud tops are very cold, a freezing temperature of -234°F, because it is so far from the Sun. It does get hotter deeper in the atmosphere since Jupiter possesses a hot iron core. In fact, like the planet Neptune, Jupiter gives off more heat than it receives from the Sun.
Jupiter has a family of 79 moons. All named after the names of lovers or daughters of the Roman god Jupiter. Its four largest was discovered by Galileo Galilei when he perfected the telescope (known as a spy glass at the time).
They are named Io, Europa, Ganymede and Callisto.
Io is the closest of the Galilean Satellites. It has more volcanoes that any other object in our Solar System. Therefore, it is geologically active. It has volcanoes erupting all the time.
Europa is made of entirely of ice. It is believed that there may be an ocean of liquid water underneath its surface. Although there has been talk about sending a probe to descend into the ice, scientists wonder is there is any life underwater.
Ganymede is by far the largest. It is almost the same size as Mars, about 3,273 miles. A super thin atmosphere of oxygen was found on Ganymede. Ganymede orbits Jupiter in about 7 days.
Callisto is the outermost of the Galilean Satellites and has whitish craters which look like that of powdered sugar! It orbits Jupiter is 17 days!
Although Jupiter is larger than all the other planets combined and it is extremely bright, it is not considered a brown dwarf or a “failed” star. A brown dwarf is a body that is at least 13 times Jupiter’s mass but is not massive enough to produce nuclear fusion within its core. If Jupiter were about 80 times more massive it could have ignited to become a star but a very small one, similar to that of our nearest star neighbor after our Sun, Proxima Centauri.
Jupiter is the first object that one can see after the Sun goes down. Search for it in the southwest towards Sagittarius. Look for the teapot and find the two stars of the handle of the teapot. There in the upper northeast you will find Jupiter. With binoculars or a telescope, you should even be able to see at least 1 or 2 of the Galilean Satellites!