Guest Author - Lea Terry

At first mistaken for a star, the planet Neptune had to wait over 200 years before its true identity was revealed.

Italian scientist and scholar Galileo Galilei was the first to spot Neptune, in 1613. He saw the planet for only two nights, however, and because he was unable to gather enough data to determine what the object was, he believed it to be a star. Then, in the 1800s, astronomers were in the midst of another planetary mystery: why Uranus’ orbit did not act in accordance with Newton’s Laws of Motion. The only reasonable explanation, it seemed, was the presence of a nearby planet whose gravitational field was influencing Uranus’ movements. Two astronomers, working independently, had calculated the unknown planet’s location by observing Jupiter, Saturn and Uranus. In 1846, this unknown planet, later named Neptune, was discovered very near the predicted location.

At nearly 17 times the size of Earth, Neptune is the fourth-largest planet in the solar system, and also one of the most remote. The eighth planet from the sun, Neptune is nearly 2.7 million miles away from the star it orbits, with an orbit that takes 165 earth years. Pluto sometimes crosses into Neptune’s orbit, and when this happens Neptune is the farthest planet from the sun for a span of 20 years once every 248 years.

Neptune may be massive, but it has the smallest diameter of the “gas giants,” a group of planets so named because they have no solid surfaces. ( Jupiter, Saturn and Uranus round out the team.) Even so, Neptune is much larger than Earth: its diameter is nearly four times the size of our planet’s, and it could hold approximately 60 Earths. Though Neptune is heavier than Earth, Earth is more dense.

Neptune seems to consist mainly of hydrogen, helium and water, in addition to silicates, the minerals largely responsible for Earth’s rocky surface. Its inner core is made of rock and ice, surrounded by heavily compressed gases that combine to form a layer of liquid. The planet’s atmosphere is mainly hydrogen and helium, with a small amount of methane. The methane is responsible for the planet’s color--when the methane absorbs red light, it produces the planet’s characteristic blue hue. Neptune is surrounded by rapidly moving clouds that race around the planet at about 700 miles per hour.

Neptune has 13 known satellites, the largest of which is Triton, measuring about 1,681 miles in diameter and located nearly 220,440 miles from the planet. Triton orbits Neptune every six days, and is the only major satellite to orbit in the opposite direction of its planet. It is also moving closer and closer to Neptune, and will likely collide with the planet in about 10 million to 100 million years. Encircling Neptune are several rings, which appear to be made of dust particles.

So far, only one spacecraft has visited Neptune: Voyager 2, which flew by in 1989. Before this, little was known about the planet, because it is so distant and difficult to observe. Though we’ve learned a great deal about Neptune, it still remains much of a mystery.







Neptune

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