Image Credit: Robert Gendler and APOD
The Pleiades (the "Seven Sisters") are one of the most widely recognized star clusters in the night sky. These days, they are rising as darkness falls in the Northern Hemisphere, and are visible most of the night. Many people mistakenly call the Pleiades the "Little Dipper," since it kind of looks like a dipper, and it is fairly small on the sky (about the size of the moon), but the Little Dipper is actually elsewhere in the sky.
The Pleiades star cluster is a fairly young star cluster, as far as astronomical ages go -- it is about 125 million years old, and hasn't even had time to orbit the Milky Way once since it formed. When the Pleiades was young, dinosaurs like this one roamed the Earth, and, if they had any astronomical inklings, they might have seen several exploding stars in the young star cluster, though that excitement has long since stopped.
Although only the seven brightest stars are visible to our eyes, the Pleiades has at least 1500 stars, and it is frequently the target of astronomical study, because even the faintest stars are relatively easy for professional telescopes to see.
From the standpoint of looking for planets, the Pleiades is a great target, Not only is it nearby, and so easy to study, but we think the sun might have been about 100 million years old when the Earth had finished forming in our Solar System 4.5 billion years ago. And some evidence suggests that the Solar System was in a star cluster when the sun formed. So, looking for evidence of forming solar systems in the Pleiades may help us understand where we came from.
Yesterday, astronomers at UCLA announced that they have found evidence of rocky (Earth-like) planets forming in the Pleiades. Their evidence, from both space-based and ground observatories, is that one star a bit bigger than our sun, called HD 23514, has a lot of dust around it. Our sun has a lot of dust around it -- most of the dust comes from comets and asteroid collisions -- but HD 23514 has a million times more dust around it than the Sun does.
Dust is a very fragile thing. Light from a star will either push dust away or cause it to spiral into the star within a few thousand years, so to see a ton of it around another star means that something has happened recently to produce it. The UCLA astronomers propose that two large planets may have collided withing the last few hundred thousand years (such a collision is what made Earth's Moon), but it could also be that there are a lot of asteroids around, many more than in our Solar System, and those could be constantly colliding and grinding each other into dust.
Whatever is going on in the Pleiades, it continues to be a great place to try and study how planets form!
And, speaking of dust, if you have insomnia this weekend and clear skies, try catching the Leonid meteors. Every 30-35 years, the Leonids put on spectacular shows with hundreds of meteors falling every minute, but this year you'll be lucky to see 10 or 20 meteors per hour. Still, the Leonids zip across the sky really quickly, and the brightest ones leave behind glowing tails that can last for several minutes! The Leonids are best seen in the Early morning hours -- you won't see very many until after midnight. Monday morning will be the best time to see Leonids, though meteor showers usually last a couple of days on either side of the peak.
The Leonid meteors are caused by dust shed by Comet Tempel-Tuttle, which orbits the sun every 33 years. So, if you see a meteor speeding across the sky early in the morning this weekend, you might be seeing a little bit of comet dust. And just imagine if you were on a planet in the Pleiades, where the meteor show would be about 1 million times more exciting!