Why is Uranus colder than Neptune if Neptune is farther from the Sun?
Uranus and Neptune are the most distant planets in the solar system;
both share similar characteristics that are classified with their category, Icy Giants,
because both have shallow temperatures. However, even though Neptune is the farthest planet,
Uranus is the coldest. Why is this happening? Let's find out!
In the solar system, we have observed that the planets closest to the sun are usually hotter
and the farthest ones colder. However, this rule is not always followed since Venus is the hottest
planet, even though Mercury is the closest. In the case of Uranus, something similar
happens since it is colder than Neptune, although the latter is a billion kilometers farther away.
While Neptune, the most distant planet in our Solar System, is 4500 million kilometers from
the Sun, Uranus, the second farthest planet, is only 2880 million kilometers away and is colder.
The atmosphere is more important than the distance While the distance to the sun is a critical
factor that determines the temperatures of the planets, probably the atmosphere is even more so;
for example, in the case of Venus, it is much hotter than Mercury because its dense atmosphere
traps heat and does not let it out, which causes a lasting greenhouse effect that keeps the surface
of the planet consistently above 480 ° C. In the case of Uranus, the atmosphere also
plays a crucial role in understanding why it is colder than Neptune.
Uranus has the coldest planetary atmosphere in the solar system, with a minimum temperature of 49 K
(-224 °C). It also has a very complex cloud structure, arranged by levels,
where it is believed that the lowest clouds are composed of water and the highest of methane.
Although there is no well-defined solid surface inside Uranus, the outermost part
of Uranus' gaseous envelope, accessible by remote sensing, is called the atmosphere.
The composition of this atmosphere is different from that of the rest of the planet, as it
consists mainly of molecular hydrogen and helium. Also, the internal heat of Uranus appears to be
lower than that of the other giant planets; in astronomical terms,
it has a low thermal flux. It is not yet clear why Uranus' internal temperature is so low.
One of the hypotheses for this discrepancy is that when Uranus received the impact it
caused its high axial tilt, the event caused it to expel most of its primordial heat,
depleting its core temperature. Another hypothesis is that there is some barrier
in the upper layers of Uranus that prevents heat from the core from reaching the surface.
For example, there may be convection in a set of layers of different compositions,
which inhibits the transport of heat upwards, causing Uranus to lose heat
very slowly compared to the rest of the planets. Uranus' atmosphere is remarkably smooth compared
to the other gas giants at ultraviolet and visible wavelengths, especially
if we compare it to its twin Neptune. When Voyager 2 flew by Uranus in 1986,
it observed ten cloud forms on the entire planet. One proposed explanation for why there are so few
is that the low temperature of Uranus prevents too many mixtures of gases at different temperatures,
as happens in Jupiter or Saturn, where the temperature differences between the
cloud layers are substantial, and that produces hurricanes and eddies that can last for centuries.
While on Uranus, there are not many temperature variations; in fact, the lowest temperature
recorded in the tropopause of Uranus is 49 K, making Uranus the coldest planet in the solar
system, and at the same time, in the calmest, in atmospheric terms, of all the gas giants.
In 1986 Voyager 2 discovered that the visible southern hemisphere of Uranus
could be subdivided into two regions: a bright polar cap and dark equatorial bands.
However, in the 90s, the number of observed bright cloud forms grew considerably, partly
thanks to the availability of new high-resolution image processing techniques. It was observed that
most appeared in the northern hemisphere, which is thought to be due to their tilt.
For its part, Neptune, orbiting so far from the Sun, receives very little heat.
Its surface temperature is −218 °C (55 K). However, the planet appears to have
an internal source of heat. It is thought that it may be a remnant of the heat produced by the
concretion of matter during its creation, which now radiates heat slowly into space.
This internal heat source produces robust climate systems around the planet, such as the Great Dark
Spot that the Voyager 2 probe discovered as it passed through the Neptune system in 1989.
Another theory suggests that in the depths of Neptune, there are ideal conditions for carbon
atoms to combine into crystals, releasing heat in the process. Therefore, this hypothesis raises the
possibility that in Neptune rain diamonds. Neptune's color differs from that of Uranus
because of the amount of helium in its atmosphere, which is slightly greater. Because of this,
Neptune absorbs more red light from the Sun than its neighboring planet,
reflecting a much more intense blue. Neptune's atmosphere has a band structure
similar to the other gas giants. Phenomena such as giant hurricanes, with a diameter equal to
that of the Earth, and other cloud formations, including some extensive cirrus clouds above
(50 km) the main clouds, occur on this planet. Thus Neptune has a very active cloud system,
possibly more active than Jupiter's. Also, the wind speed in Neptune's atmosphere is up to 2000
km/h, the highest in the solar system, and is believed to be fed by internal heat flow.
Neptune's atmosphere is 80% hydrogen and 19% helium at high altitudes.
A small amount of methane is also present, this causes prominent methane absorption
bands at wavelengths above 600 nm in the red and infrared portion of the spectrum.
Neptune's meteorology is characterized by very dynamic storm systems, with winds
reaching almost 600 m/s (2200 km/h) and almost reaching supersonic flow speeds. More typically,
by tracking the movement of persistent clouds, wind speeds have been shown to vary from 20 m/s
in the easterly direction to 325 m/s to the west, a gigantic difference that produces hurricanes
more intense than those of Jupiter. At the top of the clouds, the prevailing wind speed ranges
from 400 m/s along the equator to 250 m/s at the poles. But in general, most Neptune winds move in
a direction opposite to the planet's rotation, which is similar to what happens on Venus
Neptune is a much more active planet than Uranus, as it has an internal heat source that causes
more storms, winds, and a constant exchange of temperatures between internal and external clouds.
In contrast, Uranus has a much calmer atmosphere, without as many temperature exchanges,
so it does not have as many storm clouds or hurricanes as the rest of the gaseous planets.
The tilt of both planets If Uranus is known for something, it is for being
the only planet in the solar system that is almost totally inclined to the Sun at an angle of 97°;
This produces changes in the seasons in a completely different way than the other
major planets. The rotation of other planets can be visualized as spinning tops inclined
concerning the solar system plane, while Uranus rotates like a ball rolling inclined.
When the solstices of Uranus approach, one pole continuously faces the Sun while the other is
in the opposite direction. Only a narrow band around the equator experiences a rapid day and
night cycle, with the Sun very low above the horizon as in Earth's polar regions.
On the other side of Uranus' orbit, the poles' orientation in the Sun's direction
is reversed. Each pole receives about 42 years of uninterrupted sunlight, followed
by 42 years of darkness—a world where you would have to wait 42 years to see the sunlight again.
This inclination causes the side of the planet where it is a night not to reach a single ray of
sunlight for a long time, and because of this, the planet's temperatures drop considerably.
In contrast, Neptune has an inclination angle of only 28 °, which gives it more excellent
stability in its annual seasons because, like the Earth, the only areas where it receives
less heat are at the poles. Still, the rest of the planet is always illuminated by the Sun.
In conclusion, we can determine that the reason why Uranus is colder than Neptune is 1:
For its atmosphere, 2: for its inclination, and 3: for its core.
1: As we can see, the atmospheres of both planets are entirely different, and the main difference is
that Neptune is much more active, while Uranus is very quiet and almost without storm clouds.
2: The 97° tilt of Uranus causes extreme weather on its surface, causing one of its hemispheres
to point at the Sun for 42 uninterrupted years, while the other hemisphere remains
in total darkness for 42 years; this causes its temperatures to drop and be even colder.
In contrast, Neptune has a relatively normal inclination,
so it does not undergo the same abrupt changes in temperature that Uranus suffers.
3: And finally, perhaps the most crucial reason Uranus is colder than Neptune
is that the latter has an internal heat source in the core, while Uranus does not.
The reasons why Uranus lost this heat source are still up for debate, but so far, the most
accepted theory is that in the past, billions of years ago, a vast body collided with Uranus;
this impact on a planetary scale caused the tilt axis of Uranus to remain as it
is today and also made the heat of its core dissipate faster—Cooling the planet quickly.
It is likely that if Uranus had a hot core like Neptune's, it would also have a higher
temperature, and in that case, the coldest planet would be Neptune and not Uranus.
For now, we do not know, but perhaps in the future,
with new research and spacecraft that visit this planet, we will be able to discover all
the mysteries about these two planets that have more in common than it seems.
Today it is thought that most exoplanets in the galaxy are very similar to Uranus. Do you think we
should make an effort to investigate this planet more? Let us know your opinion in the comments!