BAN #232: The 2nd half of 2020, a planet for AU Mic

[Spiral Galaxy M81 image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona]

Number crunching

Because I think math is cool, and I think that because it is

Happy July 2nd! 2020 is a leap year, so it has 366 days. Today is the 184th day of the year, which means 2020 is officially half over, and today is the first day of the 2nd half.

What will you do in the remaining 182 days?

By the way, there are 124 days until November 3, 2020. Hopefully, in the 58 days after that until the end of the year I will be waking up on every one of those days with a big sense of relief… with just a wee bit held back until January 20, 2021.

Astro Tidbit

A brief synopsis of some interesting astronomy/science news that may be too short for the blog, too long for Twitter, but just right (and cool enough to talk about) for here.

I’m pleased to hear that astronomers have announced the discovery of a planet orbiting the very nearby red dwarf star AU Microscopii!

AU Mic is a wee bit of a star, a red dwarf just 32 light years away. It’s also extremely young, just 22 million years old. It’s still surrounded by the disk of material from which it formed… and now we know that stuff is making a planet too.

[Artwork depicting the planet AU Mic b, orbiting a red dwarf just 22 light years from Earth. Credit: NASA/GSFC]

The planet was found using the transit method, where we see the planet’s orbit edge-on, so once per orbit it passes directly in front of the star, making a mini-eclipse. This is  usually easier for smaller stars like red dwarfs because for a given size planet more of the star is blocked, making a bigger dip in light.

Except… AU Mic is a feisty little beast. Young red dwarfs are very active magnetically. They have huge sunspots that can cover much of their face and they blast out powerful flares that can actually be far brighter than the star itself. This makes measuring their varying brightness extremely tricky. Any transit may be a smaller effect than these, so it’s hard to spot.

[AU Mic (center), a dim red dwarf star just 32 light years from Earth. Credit: DSS/Skyview]

Still, the astronomers were able to see it in TESS data. They actually missed one transit because at that time TESS was doing a data dump, sending all its observations to Earth! That sucked, but they were able to see two transits (plus a third using Spitzer Space Telescope before it retired), which is enough to work with. They also got radial velocity data. You can read more about that method here, but in a nutshell a planet and star orbit around their center of mass, and so every time the planet makes one wide orbit around the center of mass the star makes a little circle. That induces a periodic Doppler shift in its light, which in principle can be measured.

The transit gives you the size of the planet, and the radial velocity the mass. What they found is that the planet, called AU Mic b, is about 1.1 times the size of Neptune (0.4 times Jupiter’s) and has a mass of 3.4 Neptunes (0.18 Jupiter). So it’s a dense sucker. It orbits just 10 million kilometers from the star, and has a period (year) of 8.5 Earth days.

I have to wonder if it’s dense because the constant bombardment from the stellar flares and the stellar wind from AU Mic (which is 1000 times the rate of the Sun’s solar wind!) are blowing the atmosphere away. It doesn’t seem old enough for that to have gotten very far yet, though, so I’m just speculating.

I’ll note that there is evidence in the data for a second planet, about 0.6 times the size of Neptune orbiting the star every 30 days or so. The data are a little ratty, but hopefully follow-up observations can confirm or refute it.

It’s a little tempting to throw this discovery on the pile of other planets, since we’ve found so many. A couple of things make this special to me though. One is that AU Mic is close, and nearby planets may be easier to study in the future with ‘scopes like James Webb. Another is that giant planets are rare around red dwarfs; they tend to make far more planets like Earth than like Neptune. So this is an interesting exception.

Also, my old friend the late Bruce Woodgate loved this little star, and observed it with STIS, a camera he designed for Hubble and which I worked on for years. He used to talk to me very excitedly about the star; the disk was visible to STIS and in fact observations taken over time showed clumps of material moving outward from the star. I’m not sure even now they’ve been adequately explained.

Anyway, Bruce would’ve been cackling with glee over this discovery. He loved astronomy and the sense of wonder it instilled. I was always happy when he would stop by my desk to talk about one thing or another we observed or could observe with STIS. I miss that sometimes.

But the world spins on, and the galaxy in which we all reside does too. So we keep looking out, keep discovering, and keep finding new and wondrous things to delight us. Even whole worlds, nearby on a cosmic scale.

Et alia

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