The Ultimate Guide to Observing Uranus, Neptune, and Pluto
January 6, 2021
Beyond the orbit of Saturn in the cold and dark depths of the outer Solar System lie the giant planets Uranus and Neptune, along with a dwarf planet and the former ninth planet in the Solar System, Pluto. While they may not get as much attention as the more popular planets closer to the Sun, each is unique in its own mysterious way.
In recent years, as science has evolved, these mysterious worlds have begun to shed some of their secrets. Let’s dive into what we know about these distant worlds and ways to observe them.
Uranus is the seventh planet from the Sun and third-largest planet in our Solar System. It was discovered in 1781 by British astronomer, William Herschel, who was mapping the night sky while looking for comets through his telescope. He came across a greenish object that was clearly a small disc but did not take on the familiar characteristics of a star. After noting its position, he returned to this mysterious object sometime later and noticed it had moved from where it should have been.
Once its orbit around the Sun was determined, Herschel was recognized for officially discovering a new planet! He named this remote world Georgium Sidus or George's Star after King George III, but the scientific community accepted astronomer Johann Elert Bode's suggestion to name it Uranus, after the Greek god of the sky. Uranus became the first planet to be discovered through a telescope as Mercury, Venus, Mars, Jupiter, and Saturn were bright enough to be seen and discovered with the naked eye.
- 5 times the mass of Earth and one of the least dense objects in our Solar System.
- Approximately 31,518 miles in diameter.
- Average distance 1.784 billion miles away from the Sun.
- Takes 17 hours to rotate on its axis and orbits the Sun once every 84 years.
- Averages 7 years to pass through each constellation of the zodiac.
- The coldest temperature found in Uranus' lower atmosphere is a frigid minus 371 degrees F.
- Rotates east to west but is unique in that it rotates on its side with an axial tilt of 98 degrees. According to NASA, the planet's north pole experiences 21 years of nighttime in winter, 21 years of daytime in summer and 42 years of day and night in the spring and fall.
- Shines at magnitude 5.7, which places it at the limited range of naked eye visibility from a dark sky site.
- At last count, Uranus has 27 moons, which are named after characters from the works of William Shakespeare and Alexander Pope.
- Uranus’ mantle is composed of water, methane, and ammonia fluids above a small core of ice and rock. Its atmosphere is made of hydrogen and helium like Jupiter’s and Saturn’s, but also contains methane. Methane gas makes Uranus appear aqua-blue. Its rocky core is proportionally larger than the gas it contains—unlike Jupiter and Saturn, which are composed mostly of hydrogen and helium and have small cores of ice and rock. This is why Uranus is now called an ice giant instead of a gas giant.
- Uranus has 13 rings, many of which are thin and dark with some wide separations. Its outer rings are surprisingly very bright and colorful. Its red ring may consist of particles reflecting red light or may possibly be composed of iron. A recently discovered outermost ring is bright blue in color.
- NASA’s Voyager 2 is the only spacecraft to visit Uranus, which it did back in 1986. The spacecraft came within 50,600 miles of the planet's cloud tops, discovered 10 new moons, two new rings, and a magnetic field stronger than Saturn’s.
Uranus will spend all of 2021 in the constellation Aries and will have a peak magnitude of +5.7. It can be spotted in a good pair of binoculars or telescope as a tiny aqua colored dot and will be visible in the morning and evening skies during these periods:
Evening: From January 1 to April 12.
- Uranus and Mars in conjunction on January 21
Morning: From May 16 to November 3
Evening: From November 4 to December 31 (end of calendar year).
- Uranus reaches opposition on November 4. This is the best time to view and image the planet.
Neptune is a dark, cold, and extremely windy world that is very similar to Uranus. It is known as the second blue planet after Earth, the eighth planet from the Sun, and the fourth-largest planet in our Solar System. Neptune is now the most distant outermost planet following Pluto’s reclassification as a dwarf planet. It is the only planet in the Solar System not visible to the unaided eye.
Neptune was discovered in 1846, the first planet discovered after mathematical calculations were used to predict its approximate location. After observing Uranus for some time, astronomers noticed that the planet was not where they predicted it would be in the night sky. French astronomer and mathematician, Urbain Le Verrier, hypothesized that this discrepancy could be due to the influence of an unseen planet. Using only his knowledge of celestial mechanics and mathematics, Le Verrier correctly predicted Neptune’s existence and position! However, German astronomer, Johann Gottfried Galle, and British mathematician and astronomer, John Couch Adams, also received credit for Neptune’s discovery as they had also worked independently to help discover this new, remote world. Neptune was named after the Roman god of the sea, as suggested by Le Verrier.
- 17 times the mass of Earth and slightly more massive than its near twin outer neighbor, Uranus, although it is somewhat smaller due to its higher density.
- Approximately 30,599 miles in diameter which equals about 4 Earths side to side.
- Average distance 2.8 billion miles away from the Sun.
- Rotates on its axis once every 16 hours and orbits the Sun once every 165 years.
- Its axis of rotation is tilted 28 degrees, so Neptune experiences seasons like on Earth—but each season lasts over 40 years!
- As the furthest planet from the Sun, Neptune is extremely frigid at minus 392 degrees F!
- Neptune shines near magnitude 8, which is beyond the limited range of naked eye visibility.
- Has 14 known moons. Triton is the only large moon in the Solar System that orbits in the opposite direction (retrograde) of Neptune’s rotation.
Similar in chemical composition to Uranus, Neptune’s mantle is composed of water, methane, and ammonia fluids above a small core of ice and rock. Its atmosphere is made up of hydrogen and helium like Jupiter and Saturn, but it also contains methane. Methane gas gives Neptune its bluish appearance. Its rocky core is proportionally larger than the gas it contains—unlike Jupiter and Saturn, which are composed mostly of hydrogen and helium and have small cores of ice and rock. Like Uranus, it’s now classified as an ice giant, not a gas giant.
- Neptune is Solar System's windiest world. Winds blow clouds of frozen methane across the planet at speeds of more than 1,200 mph in a direction opposite to the planet's rotation and faster than the speed of sound on Earth.
- Neptune features a “Great Dark Spot,” a swirling storm system the size of Earth. In recent years, the storm has been acting strangely. Storms there normally form, move to the planet’s equator, and dissipate. But the Great Dark Spot migrated towards the equator, did an about-face, and headed back towards the polar region!
- Neptune has at least five rings named after astronomers who made important discoveries about the planet: Galle, Le Verrier, Lassell, Arago, and Adams. They are composed of cold, dark compounds.
- NASA’s Voyager 2 is the only spacecraft to visit Neptune, which it did in 1989. It flew 3,000 miles above Neptune’s north pole and discovered five moons, four rings, and the "Great Dark Spot."
Neptune spends all of 2021 in the constellation Aquarius and will have a peak magnitude of +7.8. Its small bluish dot is best seen in a telescope using high magnification. It will be visible in the morning and evening skies during these periods:
Evening: January 1 through February 23
Morning: March 27 through September 13
Evening: September 14 to December 31 (end of calendar year)
- Neptune reaches opposition on September 14 and will be the best time to view and image the planet.
And finally, there is Pluto. Many of us grew up learning that Pluto was the ninth and furthest planet from the Sun. This small object was immediately classified as a planet following its discovery in 1930 by Clyde Tombaugh, an American astronomer and telescope maker. Tombaugh was searching for the mysterious Planet X, as astronomers had noticed changes in Uranus’ and Neptune’s orbit that indicated another mystery object was ‘tugging’ on the planets.
Tombaugh used a telescope that was equipped with a camera that took two photographs of the sky on different days. This device, a blink comparator, rapidly flipped back and forth between the photographs. Far away stars did not move in the images, but closer objects could be seen by its motion in the sky. After analyzing hundreds of thousands of stars, Tombaugh noticed one “star” that had moved slightly in a pair of photographs. After studying the object, the staff at Lowell Observatory in Arizona officially announced the discovery of the ninth planet. Pluto was named after the Roman god of the underworld—the equivalent of Hades in Greek mythology—due to its enormous distance away from the Sun.
For 75 years, Pluto was hailed as the ninth planet of the Solar System. But then a new trans-Neptunian object, Eris, was discovered in 2005. Astronomers around the world debated whether to make Eris the tenth planet in the Solar System. In light of the discovery, the International Astronomical Union (IAU) met in 2006 to officially define the term “planet.” At that meeting, they also created a new term, “dwarf planet,” which they applied to both Eris and Pluto. So, Pluto was demoted from a planet to a dwarf planet, a decision that remains controversial among Pluto fans to this day.
The New Horizons spacecraft put Pluto into the headlines again in 2015. This very successful flyby made discoveries that surprised scientists and transformed our understanding of this tiny, mysterious world and the dark, cold, Kuiper belt region believed to be filled with numerous asteroids, comets, and other icy worlds. For more about the New Horizons mission, check out the facts below.
- Pluto is the first Kuiper belt object to be discovered and is the largest known plutoid.
- Pluto is smaller than the Earth's moon. Its mass is .2% of Earth's mass.
- Pluto’s diameter is 1,473 miles, about 18.5% that of Earth. Pluto is only about half the width of the United States.
- Pluto’s orbit around the Sun is somewhat egg-shaped. At its closest point to Sun, Pluto is approximately 2.8 billion miles away. At its furthest point, it is approximately 5 billion miles from the Sun.
- One day on Pluto lasts about 153 hours while a whole trip around the Sun is completed in about 248 years. That means it has yet to celebrate its first birthday since its discovery!
- For 20 years, from 1979 to 1999, Pluto was closer to the Sun than Neptune.
- Pluto is a very inhospitable and cold world at minus 388 degrees F.
- Five moons have been discovered orbiting Pluto: Charon, Styx, Nix, Kerberos, and Hydra. Charon was discovered in 1978 at the U.S. Naval Observatory in Flagstaff, Arizona. It is the largest moon with a diameter just over half the size of Pluto and is bigger than the dwarf planet Eris.
- Pluto has no known ring system.
- Pluto has been visited by only one spacecraft: New Horizons, which launched in 2006 and made its historic flyby of Pluto in July 2015. Its onboard instruments gave us our first closeup images as it studied the atmosphere and surface of both Pluto and its moon, Charon. One of the most surprising discoveries was a large heart-shaped region of ice on Pluto’s surface. Also discovered were mountains made of water ice, a large fissure on Charon, and the fact that the north pole was covered with reddish material that had escaped from Pluto’s atmosphere. The four smaller moons of Pluto were found to be spinning very rapidly, unlike other moons in the Solar System.
- The discoverer of Pluto, Clyde W. Tombaugh, had an ounce of his cremated remains placed aboard the New Horizons spacecraft as a fitting tribute.
Dwarf planet Pluto will be residing in Sagittarius in 2021 with a visual magnitude of +14.4. It will be visible in the morning and evening skies during these periods:
Morning: Late January through Early June
- Venus and Pluto will be in conjunction on January 28, 2021. (Please note that while Venus and Pluto will only be about 8 degrees above the horizon at sunrise, it will be impossible to view Pluto in the brightening dawn sky.)
Evening: Late July through Early December
Helpful Observing and Imaging Tips
Uranus, Neptune, and Pluto can be much more challenging to find than the brighter naked-eye planets Mercury, Venus, Mars, Jupiter, and Saturn. The key is knowing when and where to look and using the right telescope. Although it is possible to locate Uranus without a telescope or binocular, planetarium programs such as Celestron’s Starry Night or CPWI software can help guide the way. You can also refer to astronomy-related magazines for the latest up-to-date sky maps that can help you locate these elusive targets among a sea of stars.
Modern day computerized telescopes can work with mobile apps or come with hand controls pre-programmed with tens of thousands of celestial objects in their databases, including Uranus, Neptune, and Pluto. After conducting a successful alignment, you can command your telescope to find (go-to) these outer planets. How rewarding it is to let your telescope do the hard work while you stand back and then enjoy the views!
Because Uranus is relatively bright, using a telescope with at least four inches of aperture or more at about 150x magnification should be enough to reveal its very tiny aqua-blue disc in calm skies. However, do not expect to see anything but a featureless greenish dot. Not even its faint rings will be visible. If you have access to at least an 8” aperture telescope, you might be able to see two of its brighter moons, Oberon (magnitude 14.1) and Titania (magnitude 13.9). Umbriel (magnitude 14.5) and Ariel (magnitude 14.8) will be more of a challenge as they lie closer to the planet and will be difficult to see in the planet’s glare.
To catch a glimpse of Neptune, you’ll need a telescope of at least eight inches of aperture at about 100x to 150x magnification. With equipment like this, you’ll still need steady skies to observe this tiny bluish disc. As with Uranus, do not expect to see any surface features or its faint rings. Neptune appears as a small bluish dot hanging out in a sea of background stars. It is possible to observe its largest moon, Triton (magnitude 13.5), so having access to a large aperture telescope will always be helpful.
Observing Pluto is the ultimate challenge. It is smaller than Earth’s moon and is approximately 3.3 billion miles away from us. You will need a large aperture telescope of at least eleven inches. Observing from dark skies is highly recommended. Having a detailed and accurate sky chart or planetarium program like Starry Night will be very helpful to locate this dwarf planet, although many faint stars are not depicted accurately on paper charts.
A computerized telescope will slew to the area where Pluto is located, but because it is so small and far away, Pluto will never reveal itself as a disc but only as a faint star that is indistinguishable from any other nearby faint star. Looking for nearby star patterns can be helpful in locating Pluto—especially when looking at the same field again on another night to see if a “star” had moved which can be painstakingly difficult. Those who were successful identifying Pluto sketched the field over two successive nights and compared the fields. Why not give it a try? If you are successful in identifying Pluto, you can earn bragging rights as a member of the unofficial Pluto Observers’ Club!
With camera technology rapidly evolving, amateur astronomers have been taking advantage of the latest technological advances and imaging techniques to capture the planets in unprecedented detail that was before almost impossible to achieve–including detecting atmospheric markings on Uranus and Neptune through very large amateur telescopes. While you may not have access to very large aperture telescopes, it is still possible to image Uranus and Neptune and to capture their distinct colors using smaller aperture telescopes right from your own backyard.
The camera takes the place of the telescope’s eyepiece and connects through a USB cable to a computer or laptop. Software analyzes each frame of the live video capture and throws away the images that are blurry due to atmospheric turbulence. It then stacks and perfectly aligns the clearest video frames to create a colorful image. The image scale will be small, so using a Barlow lens is recommended. If you are successful, you will have the personal satisfaction of capturing these two far away worlds, regardless how small they may appear. Celestron’s planetary imaging cameras include:
We hope this guide helped you get better acquainted with the outer planets Uranus and Neptune, and everyone’s favorite dwarf planet, Pluto. While they may not be as popular as their brighter and more well-known inner neighbors, Uranus, Neptune, and Pluto are nonetheless prominent members of the Solar System and can still be glimpsed in large amateur telescopes if you know where to look and how to locate them.
They may not reveal a lot about themselves, but the distinct colors of ice giants Uranus and Neptune are certainly worth the effort it takes to find them. And just seeing Pluto change position among the background stars is the ultimate achievement that can help you relive the feeling that Clyde Tombaugh must have felt when he first noticed Pluto’s movement among the stars.
Clear skies, good luck, and happy observing!
Other articles you might be interested in: Ultimate Guide to Observing the Universe