Extraterrestrial sky

In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth.

The skies of Venus, Mars and Titan have been observed by space probes designed to land on the surface and transmit images back to Earth.

The atmosphere's density and chemical composition can contribute to differences in color, opacity (including haze) and the presence of clouds.

Though a terrestrial observer would find a dramatic decrease in available sunlight in these environments, the Sun would still be bright enough to cast shadows even as far as the hypothetical Planet Nine, possibly located 1,200 AU away, and by analogy would still outshine the full Moon as seen from Earth.

For a Mercurian observer, on the other hand, Venus is closest when it is in opposition to the Sun and is showing its full disk.

Being closer to the Sun, Venus receives about 1.9 times more sunlight than Earth, but due to the thick atmosphere, only about 20% of the light reaches the surface.

Mercury would also be easy to spot, because it is closer and brighter, at up to magnitude −2.7,[5] and because its maximum elongation from the Sun is considerably larger (40.5°) than when observed from Earth (28.3°).

)[10] The Moon's atmosphere is negligibly thin, essentially vacuum, so its sky is black, as in the case of Mercury.

Furthermore, the Sun is so bright that it is still impossible to see stars during the lunar daytime, unless the observer is well shielded from sunlight (direct or reflected from the ground).

Lunar observers with telescopes might be able to discern the umbral shadow as a black spot at the center of a less dark region (penumbra).

Twilight lasts a long time after the Sun has set and before it rises because of the dust high in Mars's atmosphere.

On Mars, Rayleigh scattering is usually a very weak effect; the red color of the sky is caused by the presence of iron(III) oxide in the airborne dust particles.

Dust absorbs blue light and scatters longer wavelengths (red, orange, yellow).

[citation needed] Further down into the atmosphere, the Sun would be obscured by clouds and haze of various colors, most commonly blue, brown, and red.

[32] The sky in the upper reaches of Saturn's atmosphere is blue (from imagery of the Cassini mission at the time of its September 2017 demise), but the predominant color of its cloud decks suggests that it may be yellowish further down.

Observations from spacecraft show that seasonal smog develops in Saturn's southern hemisphere at its perihelion due to its axial tilt.

It seems likely that Saturn is permanently invisible behind orange smog, and even the Sun would be only a lighter patch in the haze, barely illuminating the surface of ice and methane lakes.

[33] With its thick atmosphere and methane rain, Titan is the only celestial body other than Earth upon which rainbows on the surface could form.

The other irregular outer moons would not be visible to the naked eye, although a dedicated telescopic observer could potentially spot some at full phase.

Triton's rotation axis is inclined 130° to Neptune's orbital plane and thus points within 40° of the Sun twice per Neptunian year, much like Uranus's.

Due to its eccentric orbit, Nereid would vary considerably in brightness, from fifth to first magnitude; its disk would be far too small to see with the naked eye.

A trans-Neptunian object is any minor planet in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune, 30 astronomical units (AU).

From Pluto, the Sun is point-like to human eyes, but still very bright, giving roughly 150 to 450 times the light of the full Moon from Earth (the variability being due to the fact that Pluto's orbit is highly elliptical, stretching from just 4.4 billion km to over 7.3 billion km from the Sun).

Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide gases, all of which are derived from the ices of these substances on its surface.

This atmosphere also produces a noticeable blue haze that is visible at sunset and possibly other times of the Plutonian day.

The view of outer space of exoplanets can be extrapolated from open source software such as Celestia or Stellarium.

[41] At Barnard's star the Sun would appear between the not much shifted Sirius and Belt of Orion compared to in the sky of Earth.

Planets of the TRAPPIST-1 system would appear in the sky with angular diameters comparable to the moon as viewed from Earth.

Under clear viewing conditions, details such as phases and surface features would be easily visible to the naked eye.

Furthermore, because of the LMC's high galactic latitude, an observer there would get an oblique view of the entire galaxy, free from the interference of interstellar dust that makes studying in the Milky Way's plane difficult from Earth.