(Intro)
 
Our galaxy might be more crowded than we thought! Last week, the Gaia space observatory released its first map of the Milky Way, which is on its way to becoming our most detailed map of the galaxy ever.
 
Gaia was launched by the European Space Agency in 2013, with the goal of creating a three-dimensional map of more than one billion stars and objects in our galaxy. The observatory consists of two telescopes, angled 106.5 degrees apart. It spins constantly, making one rotation about every six hours. By comparing scans that were taken six months apart, researchers can then calculate a star's distance using trigonometry, based on how the stars shift compared with the rest of the sky.
 
Gaia's detector is almost a billion square pixels, so it can pick up on faint details that have never been seen before. And so far, the telescope has already found 400 million stars that we had no idea existed. Which means that the Milky Way may be even larger than we expected. The observatory is expected to do a lot more as its five-year mission continues, and it's already settling some astronomical disputes - like the debate about how far the Pleiades star cluster is from Earth.
 
Data from the Hubble Space Telescope suggested that it was about 440 light years away, but the Hipparcos satellite, an ESA satellite launched in 1989, said that it was only about 391. Turns out that Hubble's measurement was probably more accurate. Gaia's preliminary results found Pleiades to be somewhere around 437 light years away, give or take about 20 light years.
 
The Hipparcos measurement might have been wrong because too many adjustments were made to account for the brightness of the star cluster. By the end of the Gaia mission, we should be able to calculate the distance of many of our galaxy's stars with incredible precision.
 
But the observatory will do much more than help us just  pinpoint stellar distances. It'll also improve our understanding of the Milky Way's structure, examine the distribution of dark matter in our galaxy, and help us improve our estimates of how fast the universe is expanding. And while it's scanning the skies, Gaia will find thousands of new exoplanets and track about 350,000 asteroids.


A bit closer to home, scientists have solved one of the mysteries of Pluto's moon Charon: why an area around the moon's north pole, called Mordor, is red, and it unfortunately has nothing to do with Sauron. When New Horizons flew past Pluto last July, it also took detailed images of Charon, Pluto's largest moon.
 
Unlike the rest of the moon, Charon's north pole is reddish-brown, so the New Horizons team informally named the region Mordor, after the evil, desolate place in Lord of the Rings. Charon's pole is about the same color as Pluto, and according to new research, that's no surprise: In a NASA blog post earlier this month, New Horizons scientists reported that Pluto is likely sharing its atmosphere - and therefore its color - with its largest neighbor.
 
Pluto gets its color from large organic molecules called tholins. Tholins don't occur naturally on Earth, but they are often found in the outer solar system. On Pluto, they're likely caused when cosmic radiation from outside of the solar system and UV rays from the sun react with the methane in Pluto's atmosphere and on its surface. Pluto's atmosphere is thin and wispy, and extends far from its surface, at least 160 km.
 
Since Pluto and Charon are only about 20,000 kilometers apart, the new research suggests that some of Pluto's atmosphere drifted over and got trapped by Charon's gravity at its north pole. Now, Charon's north pole is much colder than Pluto's atmosphere, with temperatures between about -260 and -210 degrees Celsius. Meanwhile, Pluto's atmosphere is about -170 degrees Celsius at 10 kilometers above the surface, and it only gets warmer from there.
 
So when gas from Pluto arrives at Charon, it immediately freezes. Then, those cosmic and UV rays interact with all the frozen material and create tholins. Over millions of years, the pole has built up a huge deposit of them, leading to its reddish-brown color. Tholins around the solar system come in lots of different colors, and the New Horizons team is still studying why the ones on Pluto and Charon are specifically reddish-brown. So even though it's been more than a year since New Horizons did its flyby, we are still learning more about the Pluto system.
 
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