Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL

What is the Surface Temperature of Neptune?

21 May , 2016

by

Our Solar System is a fascinating place. Between its eight planets and many dwarf planets, there are some serious differences in terms of orbit, composition, and temperature. Whereas conditions within the inner Solar System, where planets are terrestrial in nature, can get pretty hot, planets that orbit beyond the Frost Line – where it is cold enough that volatiles (i.e. water, ammonia, methane, CO and CO²) condense into solids – can get mighty cold!

It is in this environment that we find Neptune, the Solar System’s most distance (and hence most cold) planet. While this gas/ice giant has no “surface” to speak of, Earth-based research and flybys have been conducted that have managed to obtain accurate measurements of the temperature in the planet’s upper atmosphere. All told, the planet experiences temperatures that range from approximately 55 K (-218 °C; -360 °F) to 72 K (-200 °C; -328 °F), making it the coldest planet in the Solar System.

Orbital Characteristics:

Of all the planets in the Solar System, Neptune orbits the Sun at the greatest average distance. With a very minor eccentricity (0.0086), it orbits the Sun at an semi-major axis of approximately 30.11 AU (4,504,450,000,000 km), ranging from 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.

Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA

Neptune and the icy-asteroid-rich Kuiper Belt that lies beyond its orbit. Credit: NASA

Neptune takes 16 hours 6 minutes and 36 seconds (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).

Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. In addition, the planets axial tilt also leads to variations in the length of its day, as well as variations in temperature between the northern and southern hemispheres (see below).

“Surface” Temperature:

Due to their composition, determining a surface temperature on gas or ice giants (compared to terrestrial planets or moons) is technically impossible. As a result, astronomers have relied on measurements obtained at altitudes where the atmospheric pressure is equal to 1 bar (or 100 kilo Pascals), the equivalent of air pressure here on Earth at sea level.

In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Image: Erich Karkoschka)

Color-contrasted photo showing Neptune’s atmospheric features. Credit: Erich Karkoschka

It is here on Neptune, just below the upper level clouds, that pressures reach between 1 and 5 bars (100 – 500 kPa). It is also at this level that temperatures reach their recorded high of 72 K (-201.15 °C; -330 °F). At this temperature, conditions are suitable for methane to condense, and clouds of ammonia and hydrogen sulfide are thought to form (which is what gives Neptune its characteristically dark cyan coloring).

But as with all gas and ice giants, temperatures vary on Neptune due to depth and pressure. In short, the deeper one goes into Neptune, the hotter it becomes. At its core, Neptune reaches temperatures of up to 7273 K (7000 °C; 12632 °F), which is comparable to the surface of the Sun. The huge temperature differences between Neptune’s center and its surface create huge wind storms, which can reach as high as 2,100 km/hour, making them the fastest in the Solar System.

Temperature Anomalies and Variations:

Whereas Neptune averages the coldest temperatures in the Solar System, a strange anomaly is the planet’s south pole. Here, it is 10 degrees K warmer than the rest of planet. This “hot spot” occurs because Neptune’s south pole is currently exposed to the Sun. As Neptune continues its journey around the Sun, the position of the poles will reverse. Then the northern pole will become the warmer one, and the south pole will cool down.

Neptune’s more varied weather when compared to Uranus is due in part to its higher internal heating, which is particularly perplexing for scientists. Despite the fact that Neptune is located over 50% further from the Sun than Uranus, and receives only 40% its amount of sunlight, the two planets’ surface temperatures are roughly equal.

Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Deeper inside the layers of gas, the temperature rises steadily. This is consistent with Uranus, but oddly enough, the discrepancy is larger. Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. The mechanism for this remains unknown.

And while temperatures on Pluto have been recorded as reaching lower – down to 33 K (-240 °C; -400 °F) – Pluto’s status as a dwarf planet mean that it is no longer in the same class as the others. As such, Neptune remains the coldest planet of the eight.

We have written many articles about Neptune here at Universe Today.  Here’s The Gas (and Ice) Giant Neptune, What is the Surface of Neptune Like?, 10 Interesting Facts About Neptune, and The Rings of Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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Sort by:   newest | oldest | most voted
Patrick Moore
Guest
January 24, 2009 10:17 PM

uranus can be very itchy. maybe u have worms? I pull my trousers right up to my neck when I do sky at night.

Jona Marie Hernandez
Guest
Jona Marie Hernandez
January 29, 2009 3:26 AM

ThAnK yOu to all the people that write this article because now i know more about neptune!!!

Can you pls.write more article about the planets because i’m enjoying reading a article about the planets!!!

lloyd
Guest
lloyd
May 21, 2016 7:13 PM

Seems like it’s almost in the “Goldie Locks” zone! Imagine 40 years of summer! That’s a lot of lemonade!

The One
Guest
The One
May 22, 2016 1:12 AM

Imagine -203.15 c? That’s not quite the lemonade summer you were thinking about, is it?

Dave
Member
Dave
May 23, 2016 7:59 AM

Sweatshirt weather. I love it.

sang
Guest
May 21, 2016 9:45 PM

An incomplete article due to the following facts — 1) Neptune’s stratosphere is at least 30 degree hotter than Uranus; 2) Neptune has a great internal heat source compared with that of Uranus, i.e., it releases more heat than receiving from the sun.

gopher65
Member
gopher65
May 21, 2016 11:09 PM

sang: All the planets give off more heat than they receive from the sun. Think about it: the heat the release will be equal to one solar input (whatever that happens to be for that planet), + whatever internal heat they’re releasing.

The One
Guest
The One
May 22, 2016 1:15 AM

Why don’t they ask what the internal heating is from? I really wan’t to hear this explanation… wink

WildBill
Guest
WildBill
May 22, 2016 8:32 AM

@The One — Gravity.

Dave
Member
Dave
May 23, 2016 8:00 AM

Friction

Brian
Guest
Brian
May 21, 2016 11:08 PM

I really get tired of articles that use everything except the english system. Kilometers, celsius, kelvin, all those scales are meaningless. If you’re going to publish articles in the U.S. at least include english system measurements on every numerical data you list. Sheeesh.

gopher65
Member
gopher65
May 21, 2016 11:56 PM

Yeah. I mean, we’d hate for the 4% of the world’s population backwoodsie enough to still use outdated units to be inconvenienced. Or they could, you know. Learn.

Pvt.Pantzov
Member
May 22, 2016 8:19 PM

I wonder if he knows that American scientists who work in the fields covered on this site also use the metric system.

DrakTheDrake
Member
DrakTheDrake
May 23, 2016 10:11 AM
Why not? The values are just as meaningless regardless of the units, relatively speaking. For example, referring to the core temperatures, you would be lying if you said that 12,632 degrees F is any more meaningful to you than 7,000 degrees C. You have no frame of reference for numbers that large, when it comes to temperature, other than “really really hot.” Looking at the wind speed, which didn’t have an imperial unit conversion attached to it, Google tells me that 2100 km/h comes out to 1304.88 m/h. Again, those numbers are pretty much incomprehensible to anyone except as raw numbers. Both numbers mean “really really fast.” So what does it matter which units they use if it’s… Read more »
Marc
Guest
Marc
May 22, 2016 8:39 AM

So much “scientific” BS nowdays. Why don’t they admit they don’t know chit? I am so sick of all these theories and guesses where the people making them make sci fi authors look like absolute geniuses in comparison. Our government is over educated nerds and so is our scientific society. Where has common sense and logic gone? I guess it is out the window…….

seahawkfan76
Member
seahawkfan76
May 22, 2016 8:49 PM

Yep, and the biggest lie of all is evolution. Talk about theories and guesses! They cannot admit that evolution just cant happen, because they would lose everything. All of their work would be for nothing, and some of their work has even been faked! There are no transitional fossils which even Darwin said if we cant find, his theory would be false.

Kevin
Guest
Kevin
May 22, 2016 1:02 PM

what causes the 1500 mph winds???

Kevin
Guest
Kevin
May 22, 2016 1:03 PM

what causes the 1500 mph winds???

LordJohn
Member
LordJohn
May 22, 2016 3:39 PM

One problem with this article the picture with the caption beneath saying “Image of Neptune’s and close-up of its Dark Spot, taken by the Hubble Telescope. Credit: NASA/ESA/HST” is actually a picture of Uranus. which it says so at the top of the picture. So people will now be confused. Perahps you should take the article down, re-edit and then restore it. Or perhaps no one else noticed.

joe
Guest
joe
May 22, 2016 8:21 PM

sorry, but you’re wrong. that is neptune

Richard Campbell
Guest
May 22, 2016 7:17 PM

I JUST DON’T UNDERSTAND WHY NASA JUST DON’T SEND A SPACECRAFT TO THESE GAS GIANTS AND STUDY THEM UP CLOSE.and lets all see what they are truly like.

chumchingee
Member
chumchingee
May 24, 2016 6:23 PM
The Earth in a typical basement in an average zone such as Ohio is in, has a temperature of 52 degrees F. This temperature remains constant in the summer. The temperature at the core is what? 8,000 degrees F? That could be off a little bit. Okay, temperatures at our surface of our planet vary considerably. Possibly any where from 40 degrees F. below zero to 120 degrees F. in countries near the Equator. A lot of that is contributed by our relationship to the local Star we call the Sun. Now, if you have a core temperature as described in the article then at some point deep in the atmosphere you have a goldilocks zone on Neptune.… Read more »
yungboss
Member
yungboss
May 26, 2016 4:13 AM

interesting read

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