Extraterrestrial Skies - Extrasolar Planets

Extrasolar Planets

For observers on extrasolar planets, the constellations would be quite different. The Sun would be visible to the naked human eye only at distances below 20–25 parsecs (65–80 light years). The star β Comae Berenices is slightly more luminous than the Sun, but even over its relatively close distance of 27 light years, appears quite faint in our sky.

If the Sun were observed from the Alpha Centauri system, the nearest star system to ours, it would appear to be a bright star in the constellation Cassiopeia. It would be almost as bright as Capella is in our sky.

A hypothetical planet around either α Centauri A or B would see the other star as a very bright secondary. For example, an Earth-like planet at 1.25 astronomical units from α Cen A (with a revolution period of 1.34 years) would get Sun-like illumination from its primary, and α Cen B would appear 5.7 to 8.6 magnitudes dimmer (−21.0 to −18.2), 190 to 2700 times dimmer than α Cen A but still 2100 to 150 times brighter than the full Moon. Conversely, an Earth-like planet at 0.71 AUs from α Cen B (with a revolution period of 0.63 years) would get Sun-like illumination from its primary, and α Cen A would appear 4.6 to 7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than α Cen B but still 5700 to 470 times brighter than the full Moon. In both cases the secondary sun would, in the course of the planet's year, appear to circle the sky. It would start off right beside the primary and end up, half a period later, opposite it in the sky (a "midnight sun"). After another half period, it would complete the cycle. Other planets orbiting one member of a binary system would enjoy similar skies.

From 40 Eridani, 16 light years away, the Sun would be an average looking star of about apparent magnitude 3.3 in the constellation Serpens Caput. At this distance most of the stars nearest to us would be in different locations than in our sky, including Alpha Centauri and Sirius.

From a planet orbiting Aldebaran, 65 light years away, the Sun would appear slightly above Antares in our constellation Scorpius, and at magnitude 6.4 would barely be visible to the naked eye. Constellations made of bright, far-away stars would look very similar (such as Orion), but much of the night sky would seem unfamiliar to someone from Earth.