Does the title of this column sound like a sci-fi novel? WISE 0855-0714 is in fact a real brown dwarf, and it’s totally plausible that snowstorms occur in its atmosphere. Let’s take a closer look at what’s happening on this strange world.
First, what exactly is a brown dwarf? It’s a gaseous object not massive enough to be a star, but much more massive than a planet like Jupiter. Its mass is estimated to be between 13 and 75 times greater than Jupiter’s. Unlike a star, a brown dwarf doesn’t generate its energy through nuclear fusion of hydrogen, but rather by contracting on itself. A brown dwarf cools over its lifespan. The hottest brown dwarfs have a surface temperature of about 2,000 °C; the coldest ones have a temperature below 1,000 °C. Brown dwarfs are therefore not very luminous in visible light. We in fact discover them through telescopes relying on infrared light.
The brown dwarf mentioned in the title was discovered in 2014 by astronomer Kevin Luhman using data from the infrared space telescope WISE. This dwarf lurks in the spring constellation Hydra. The numbers in its name refer to its approximate coordinates in the sky.
It’s located only 7.53 light-years away, making it the fourth closest star system to the Sun. Despite its proximity, WISE 0855-0714 is impossible to detect visually through an amateur telescope because in visible light, it shines only at magnitude +25. That means it’s two quadrillion times less luminous than the full Moon!
But it’s another aspect of WISE 0855-0714 that interests us here: this brown dwarf is the coldest ever discovered. Its surface temperature probably varies between –48 °C and –13 °C. Observations also suggest the presence of water clouds and a convection process on this gaseous body. Hence, its atmosphere conceivably contains precipitation, which, in such cold temperatures, could only be snowstorms.
Planets in December
Mars still reigns in the evening sky in December. The red planet is visible at twilight fairly high due south and sets in the west around 11 p.m. You can easily spot it in the sky thanks to its orangey red hue. On the evening of December 14, the first quarter Moon lies 4 degrees below the red planet and will help you identify it.
But the end of the night is the best time to see planets at the moment. First, there’s Venus, which shines brightly in late 2018. You can initially make it out above the east-southeastern horizon about three hours before sunrise; at dawn, it stands out on the southeastern horizon due to its dazzling glow. On December 3, around 4 a.m., you can observe the crescent Moon 6 degrees above Venus. The next day, the crescent is now to the lower left of Venus. A little to the upper right of Venus one may notice the star Spica in the constellation Virgo.
On December 5, an hour before sunrise, the thin crescent now lies to the upper right of Mercury. It’s an ideal occasion to observe the planet, which is often hard to spot since it always sticks close to the Sun.
Finally, Jupiter reappears in the glow of dawn as of mid-December. Around 6:30 a.m. on December 21, sneaky Mercury lays less than 1 degree to the upper left of Jupiter. This chance encounter brings fall to a close since winter in the northern hemisphere begins at 5:22 p.m. on this date.
The peak of the Geminids
The Geminid meteor shower hits its peak on December 14 at 7:30 a.m. The best night for observations is therefore between December 13 and 14 after the crescent Moon sets around 10 p.m. In favourable conditions, under a clear sky far from light pollution, you may see up to 60 meteors an hour, especially just before dawn when the constellation Gemini shines very high due south.
A plethora of planets
To end the year with a bang, watch for a grouping of planets in the sky on December 31 about 45 minutes before sunrise. From left to right, you’ll find Mercury just 5 degrees above the southeastern horizon. Bright Jupiter lies a bit to its upper right, just to the left of the red star Antares in the constellation Scorpius. In the south-southeast shines dazzling Venus. Finally, higher in the south, observe the waning Moon and, a bit farther on its right, the star Spica, which marks the end of this celestial panorama.
In closing, I wish you all a lovely holiday season. May this festive period be filled with joy and the beauty of our starry sky.