Mass of Pluto

In everything but the largest telescopes, Pluto appears as a tiny dot. And determining mass from so little information is incredibly hard to do.

Astronomers could only try and work out its mass by knowing how bright it was – its albedo. They could detect that it had large quantities of methane ice on its surface, and so astronomers knew that it had to be very bright. But there were sure about Pluto’s size, or even if it was larger than Mercury or Earth’s moon.

But astronomers lucked out in 1978 when James Christy discovered Pluto’s moon Charon. Once you get a system where two objects are orbiting one another, such as in the case of Pluto and Charon, you can use Newton’s formulation of Kepler’s Law to work out the mass very precisely.

Plugging in the orbital information for Pluto and its moon Charon, astronomers calculated its mass to be 1.31 x 1022 kg – less than 0.24% the mass of Earth. Followup observations were able to determine its size very accurately as 2,390 km across.You can also look through these books from if you want more information about Pluto.

Temperature of Pluto

With such a large distance from the Sun, Pluto is incredibly cold. But this temperature can vary enough to change the dwarf planet significantly. At its closest point, it warms up enough so that Pluto’s nitrogen atmosphere sublimates and forms a diffuse cloud around it. As Pluto gets further away from the Sun; however its this atmosphere freezes out, and falls to the surface of Pluto like snow.

First, let’s define some measurements. Room temperature is considered 21-degrees Celsius or 70-degrees Fahrenheit. The freezing point of water is 0-degrees Celsius or 32-degrees Fahrenheit. But when you’re measuring temperatures on Pluto, you really want to use Kelvin.

Zero Kelvin is the absolute zero temperature; a theoretical maximum point where no more energy can be extracted from a system. 0-degrees Kelvin corresponds to -273-degrees Celsius.

The surface of Pluto, in comparison, can range from a low temperature of 33 Kelvin (-240 degrees Celsius or -400 degrees Fahrenheit) and 55 Kelvin (-218 degrees Celsius or -360 degrees Fahrenheit). The average surface temperature on Pluto is 44 Kelvin (-229 Celsius or -380 Fahrenheit).

Back in the days when Pluto was still a planet, it was the coldest planet in the Solar System. But now it’s just a regular temperature dwarf plant – poor Pluto. Neptune is now the coldest planet.

Distance to Pluto

Pluto has the most elliptical orbit of all the planets and dwarf planets. In addition to this widely varying orbital distance, Pluto is also highly inclined, orbiting above and below the planet of the ecliptic that the rest of the planets follow.

You can also look through these books from if you want more information about Pluto.

Pluto Distance from the Sun
Since Pluto orbits the Sun, like the rest of the planets and dwarf planets, astronomers typically measure the distance of Pluto in terms of Astronomical Units (AU). 1 AU measures the distance of the Earth to the Sun.

At its closest point, Pluto is only 29 astronomical units from the Sun (4.4 billion km or 2.75 billion miles). And at its most distant, it can be 49 AU (7.29 billion km, or 4.53 billion miles) from the Sun. In addition to being highly elliptical however, Pluto’s orbit is also inclined at an angle of over 17-degrees. At some points along its orbit, Pluto is above the plane of the ecliptic that the planets follow, and at other times, it’s below.

Pluto’s average distance from the Sun is 40 astronomical units (5.91 billion km or 3.67 billion miles).

Distance From Earth to Pluto
The Earth is only 1 AU from the Sun. When the Earth and Pluto are perfectly lined up with the Sun, their closest point is approximately 28 astronomical units. And at their furthest point, when Earth is on the opposite side of the Sun, Pluto can be 50 astronomical units.

Pluto Atmosphere

Artist's impression of a comet's surface. Image credit: NASA/JPL

Yes, that’s right, Pluto does have an atmosphere. Well, the Pluto atmosphere is not the ocean of air we have here on Earth, but Pluto’s thin envelope of gases do surround the dwarf planet for part of its orbit around the Sun.

You can also look through these books from if you want more information about Pluto.

It’s important to understand that the orbit of Pluto is very elliptical, bringing it closer and then more distant at various points of its orbit. At the closest point, the surface of solid nitrogen heats up enough that it sublimates – changes directly from a solid to a gas.

These clouds of nitrogen surround Pluto, but it doesn’t have enough gravity to keep them together, so they can escape out into space.

And then, as Pluto gets further from the Sun again, it cools down, and the atmosphere freezes and solidifies back down on the surface of Pluto.

In 1988, astronomers discovered that Pluto has an atmosphere by watching how it passed in front of a more distant star – called a planetary transit. Instead of dimming the moment it went behind Pluto, the star was first obscured by the atmosphere, so that astronomers could measure its thickness and composition.

It currently has 3μbar on the surface and its height extends 60 km above the surface.

More precise observations were done in 2002, when astronomers were surprised to find that Pluto’s atmosphere had actually thickened since it had first been discovered. Astronomers think this is a seasonal phenomenon. The nitrogen on Pluto’s surface was exposed to sunlight following a 120-year winter. The nitrogen became a gas, but it took time to get going as an atmosphere.

As Pluto is now traveling away from the Sun, the Pluto atmosphere won’t last long. Astronomers think it will begin to disappear by 2015. This is one of the big reasons NASA sent its New Horizons spacecraft – to study Pluto’s atmosphere before it’s gone for good.

Pictures of Pluto

Pluto is so small and distant that we just don’t have any good pictures of it… yet. We get so many people asking that I’ve compiled together a gallery of the best pictures of Pluto. Some of these are actual Pluto pictures, captured by telescopes, while others are pics of Pluto done by an artist. Once NASA’s New Horizons spacecraft finally arrives in 2015, we’ll get some actual, close up images of Pluto and its moon Charon.

Even though Pluto’s not a planet any more, we can’t wait to see what it’s going to look like.

Each image links to a version you can use as your desktop background. To do this, click on an image to see the larger version, and then right-click and choose “Set as desktop”. Now you’ll have the picture as your background.

You can also look through these books from if you want more information about Pluto.

This is one of the best hubble pics of Pluto ever taken. It was photographed by the Hubble Space Telescope in 1994. The image clearly shows both Pluto and Charon as separate disks with surface features.

This is a picture of Pluto, captured by the Hubble Space Telescope. The photograph of Pluto was taken when the dwarf planet was 4.8 billion km (3 billion miles) from Earth. Hubble was able to see lighter and darker patches across the surface of Pluto. What’s happening here? We’ll have to wait for New Horizons to know better.

This is an artist’s illustration picture of Pluto and Charon seen from one of its smaller moons. Pluto is the large disk right in the middle of the photograph, and Charon is the smaller one over to the right. Pluto’s other tiny moon is the bright object to the left, just above the horizon. (Image credit: NASA).

Here’s a new portrait of the Solar System, with tiny Pluto and the other dwarf planets. You can see how they compare in size to the rest of the planets.

This is a picture of Pluto being visited by NASA’s New Horizons spacecraft. The actual encounter is going to happen in 2015, when the first close-up images of the surface of Pluto will be sent back to Earth.

I hope you enjoyed these Pluto pics.


Take a look at the Solar System from above, and you can see that the planets make nice circular orbits around the Sun. But dwarf planet’s Pluto’s orbit is very different. It’s highly elliptical, traveling around the Sun in a squashed circle. And Pluto’s orbit is highly inclined, traveling at an angle of 17-degrees. This strange orbit gives Pluto some unusual characteristics, sometimes bringing it within the orbit of Neptune.

Pluto takes 248 years to complete one full orbit around the Sun. During this journey, the orbit of Pluto ranges in distance from the Sun following an elliptical orbit. At its closest point, it can be 30 astronomical units from the Sun (1 AU is the distance from the Earth to the Sun). At its furthest point, Pluto is 39 AU from the Sun.

Astronomers call this orbit eccentric because Pluto follows an orbit that traces out an elongated ellipse around the Sun.

Pluto’s orbit is also highly inclined. This means that it doesn’t orbit within the same plane as the rest of the Solar System. Instead, Pluto orbits at an angle of 17-degrees. For part of its orbit, Pluto is above the plane of the ecliptic (where the other planets orbit) and other times it’s below that plane.

Because the orbit of Pluto varies so widely, it can switch places with Neptune, orbiting closer to the Sun. The last time this happened was on February 7, 1979. Pluto remained closer to the Sun than Neptune until February 11, 1999. And the previous time it happened was back in the 1700s.

With its low mass, Pluto’s orbit is actually quite chaotic through its interactions with Neptune. Although astronomers can predict its position forward and backwards in time for a few million years, the uncertainties mount up, and it’s impossible to know where it’ll be in the far future.

As you probably know, Pluto is no longer a planet. This was a decision handed down in the 2006 meeting of the International Astronomical Union. Although Pluto orbits the Sun and has enough mass to pull itself into a sphere, it hasn’t cleared out its orbit.

They’ll never collide, though. Pluto is in a 3:2 resonance with Neptune. This means that for every three orbits Neptune makes going around the Sun, Pluto makes two. They always end up in the same positions. This whole process takes about 500 years to complete.

Just to give you an example, Pluto’s mass is only 0,07 times the mass of all the other material in its orbit. Earth, in comparison, has 1.5 million times the mass of everything else in its orbit.

Because it hasn’t cleared out this material, Pluto was designated as a dwarf planet, along with asteroid Ceres and the newly discovered Eris, which is actually larger than Pluto.

We have written many interesting articles about Pluto here at Universe Today. Here’s facts on Pluto.


Universe Today’s Guide to Space

The Solar System:

Outer Space:

The history of the billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions.universe starting the with the Big Bang. Image credit:

Space Exploration:

History of Space Exploration:

Space Shuttle Columbia launching on its maiden voyage on April 12th, 1981. Credit: NASA

Famous Scientists:

Einstein Lecturing

Famous Astronauts:

Apollo 11 Crew Photo. Credit: NASA

Astronomy & Cosmology:

Earth Sciences:

Compilation of all known Messier Objects. Credit: Michael A. Phillips


Messier Objects:



This artist’s rendering shows the Extremely Large Telescope in operation on Cerro Armazones in northern Chile. The telescope is shown using lasers to create artificial stars high in the atmosphere. Image: ESO/E-ELT

Image Credits:

  • Observable Universe: Pablo Carlos Budassi
  • Animated Solar System: Caltech/nagualdesign
  • Big Bang:
  • Space Shuttle Columbia: NASA
  • Albert Einstein: National Library of Austria/F Schmutzer/Public Domain
  • Apollo 11 Crew: NASA
  • Hubble Deep Field: NASA and A. Feild (STScI)
  • Messier Objects: Michael A. Phillips
  • Voyager Golden Record: NASA
  • Extremely Large Telescope: ESO/E-ELT

How Long is a Day on Saturn?


If you were on the surface of Saturn, how long would a day last? This has remained a mystery for scientists, because the thick clouds of gas obscure the surface of the planet from direct observation by telescopes or orbiters. Below all those clouds there is a surface that rotates at a constant speed. Since scientists can’t directly see the surface, they’ve taken another approach: listening.

You can also check out these cool telescopes that will help you see the beauty of planet Saturn.

With the help of radio emissions that come from the interior of Saturn, scientists have been able to close in on its rotation period. Charged particles trapped in the interior emit radio waves when they interact with Saturn’s magnetic field, at about 100 Kilohertz. It’s as if Saturn had its own radio station broadcasting at a certain frequency, and as the magnetic field deep inside the planet rotates it changes the frequency of the station.

Voyager measured these emissions for nine months when it passed by in the 1980s, and the rotation was calculated to be 10 hours 39 minutes 24 seconds, with an uncertainty of 7 seconds. The Ulysses spacecraft also monitored the emissions 15 years later, and came up with a result of 10 hours 45 minutes 45 seconds, with a 36 second margin of error.

Wait, that’s 6 minutes of difference! Either Saturn slowed down a lot over the years, or something else is going on. Cassini has been measuring these same radio emissions with its Radio and Plasma Wave Science instrument, and has observed that in addition to this long-period variation, the rotation differs by as much as one percent in a week.

Scientists think that this could be due to two different things: the solar wind coming from the Sun is interfering with the measurements, or particles from Enceladus’ geysers are affecting the magnetic field. Both of these would cause the radio emissions to vary, and they could be causing the different results simultaneously.

Cassini’s new data strongly suggests that the solar wind is a likely culprit: there is a variation in the measurements of the short-period rotation every 25 days, which corresponds with the rotation of the Sun as seen from Saturn. The speed of the solar wind, too, varies the measurements, so must be accounted for. Enceladus could be the cause of the long-term difference, but more measurements are needed to see if this is definitely the case, or if there is yet another factor.

Nailing down the rotation of Saturn will be helpful in calculating the true wind speeds of the clouds, and give important clues about the composition and distribution of the interior. Once the interference from the solar wind and Enceladus are taken into account, the true rotation of Saturn can be determined precisely.

Then only one question remains: do they have commercials on Saturn FM?

Source: ESA News Release