Which Two Planets Have Only One Moon?

Have you ever wondered which two planets have only one moon? The answer may surprise you. Here you will learn which two planets are most like Earth. They have similar orbits but are not as close to the Sun. This would normally give them a higher chance of having a moon. But Venus does not have a moon and moves in a reverse motion. It also rotates at a very slow rate. In fact one year on Venus is shorter than a day on Earth.


While Earth has a moon Venus has none. This is because Venus is a slow rotator and a day on Venus is longer than a year. Its orbit is also retrograde the opposite direction of most planets. This slow rotation makes it difficult to orbit a moon. Despite this Venus is a fascinating planet to study. Here are some interesting facts about Venus. You can learn about its moons and maybe even spot a new one!

On Wednesday April 27 the moon will pass in front of Venus as a perfect right triangle. This will be a spectacular event to watch and observers in Southeast Asia will get to see it. The moon will be about a degree or so off of the equator making the occultation of a planet even more spectacular. In addition the Moon will be at its closest since November 2004 when Venus and Jupiter first became visible.

Mercury is too close to the Sun for a moon to exist on it. In addition Mercury has a hill sphere that measures 175000 kilometers in diameter. Any celestial object that attempted to revolve around Mercury would crash into it. Venus which orbits the Sun at approximately 67 million miles would have destroyed any moon that existed there. But some theories have it that Venus once had a moon and it may have been destroyed by two major impacts.


The lack of a moon on Mercury is a puzzle that evades astronomers. While many researchers believe it’s due to the effects of solar winds and the condensation radii of lighter materials this is a theory that doesn’t hold up to the test of time. During Mercury’s formation certain trace elements such as methane and hydrogen remained in gaseous form near the Sun but were swept away by solar winds. These materials including elements such as iron nickel and phosphorus were then left in solid form which coalesced into Mercury.

Moons are common on other planets in our solar system including Earth and Mars. But Mercury which is the nearest planet to the Sun has no moon at all. This is because Mercury’s gravity is too weak to support a moon. And Mercury’s atmosphere is so thin that it does not attract a moon’s gravity. Mercury is the only planet in the solar system that doesn’t have a moon and this makes it an even more intriguing mystery.

The close proximity of Mercury to the Sun makes it extremely hot. The planet’s surface temperature can reach up to 840 degrees Fahrenheit during the day but it drops to minus 275 degrees in the evening. This is probably the largest temperature swing in the solar system. The moons are also too far from Mercury to affect its surface temperatures. However Mercury’s elongated orbit also means that Mercury is able to stay relatively stable at the same temperature year-round.


The outer rings of Uranus are dark and narrow and in 1977 they blocked the light of a distant star. Voyager 2 discovered two moons on either side of the outermost ring which herded the ring particles into a narrow formation. However when scientists reexamined Voyager data they discovered wavy patterns in two of the other outer rings. It’s possible that these wavy patterns are caused by gravitational tugs from moons outside each ring.

The number of known satellites of Uranus ranges from twenty-seven to forty-seven. The five rounded moons of Uranus each contain between 0.7% and 40% of the planet’s mass. Another four moons which collectively make up 0.1% of the planet’s mass are named after Shakespearean characters. Voyager 2’s images of Miranda show dark streaks on the surface and higher resolution images reveal ridges and valleys.

Voyager 2 was the first spacecraft to visit Uranus. The probe was only 30 years old at the time but it found little atmospheric activity. It was mostly cloudy with icy clouds. However Voyager 2 had made a flyby of Saturn during its Grand Tour of outer planets. The Voyager 2 mission was responsible for the first close-up observations of Uranus.


The Moons of Neptune are the most prominent features of the planet but there are many others too. The moons of Uranus have statuesque mountains deep valleys vast plains and mysterious dark surfaces. The size of these moons varies from 25 miles to over a thousand miles. Neptune has eight natural satellites whereas other planets have many more.

The moons of Neptune are small and their orbits are roughly 30 times farther away from the Sun than Earth is. Scientists believe that Neptune was born closer to the Sun than it is now because its water would have evaporated if it were too close. Neptune’s orbits help them understand the conditions of the early solar system. Scientists hope to learn more about these conditions in the future.

Scientists first discovered Neptune in 1613. In a drawing of the planet Galileo noted that it appeared to be a star but he did not recognize it as a planet. Other scientists like Urbain Le Verrier and John Couch Adams predicted the discovery of Planet X in the 17th century but they did not identify it as such. Scientists then continued to study it for another five centuries.

The orbits of Neptune’s three moons differ from those of its other planets. Neptune has a ring-shaped moon orbited by three planets with the elliptical moon Hippocamp circling it between them. The moons have different shapes and physical properties and scientists are still trying to figure out how they’re connected. However these differences in moon size and orbits make the planet unique.


In astronomical terms Triton is the seventh-largest moon of Neptune and one of only two planets with only one moon. Its size makes it the second-largest planetary moon relative to its primary and it is slightly larger than Pluto. Triton has a geologically active history but there are no obvious impact craters on the surface. Rather it is characterized by an intricate cryovolcanic terrain and a complicated geological history.

The surface of Triton is covered in frozen lakelike features that are terraced. These cliffs result from successive melting events where successive patches of ice are deposited over a longer time. Some of the cliffs rise one kilometer high while others are less than one kilometer high. However methane and nitrogen ices are not strong enough to sustain such heights which makes it difficult to observe in the planet’s spectra.

Although there are many strange moons in the solar system none is as mysterious as Triton. Neptune has fourteen moons and most of them are small. Of the 14 satellites in Neptune’s system Triton contains 99.7% of the planet’s mass. Its retrograde orbit is highly inclined and nearly circular which makes it a curious cosmic visitor.


The rings of Saturn are a fascinating part of the planet’s solar system. The planet has two large prominent rings and a smaller fainter ring known as the Cassini division. The rings themselves are incredibly thin and varied in size ranging from the diameter of a grain of sugar to the width of a whole house. Scientists believe that they are made of debris from comets asteroids and shattered moons. In 2016 researchers even suggested that the rings may be the carcasses of a dwarf planet.

Saturn is a gas giant composed mainly of hydrogen and helium. The planet has a volume greater than that of 760 Earths. It is the second most massive planet in our solar system and it has 95 times the mass of Earth. It is also the least dense of all the planets and is only one-third the density of water. In fact Saturn has three moons. Each one is unique in size shape and composition.

Saturn has gold bands on its surface. Scientists attribute this color to the high-speed winds blowing at Saturn’s north pole. The jet stream is approximately 20000 miles long and composed of winds reaching 200 miles per hour. A massive rotating storm lies at its center. This spiral motion is responsible for the planet’s rich atmosphere. But Saturn’s unique atmosphere has even more mysterious secrets. Its magnetic field is thought to be rooted in its metallic-hydrogen outer core.