Sunburned planet turns hot face away from star

The entire universe in blog form
Oct. 21 2010 7:00 AM

Sunburned planet turns hot face away from star

More exoplanet news, and yet another instance where the more we look at them, the weirder they get.

spitzer_upsandbSpitzer Space Telescope sees in the infrared, so in a way it can measure the heat from an object. The orbiting observatory was pointed at the star Upsilon Andromedae, one of the very first stars known to have exoplanets. One of those planets, Ups And b, orbits so close to the star that it makes a complete circle around it every 4.6 days.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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That close to the star, the planet should be tidally locked: one side always facing the star, and very hot; the other facing away, and much cooler. As the planet goes around the star, from our vantage point we see first the hot face, then the cooler one, and back again, a cycle repeated every 4.6 Earth days.

Now, we can't separate the planet from the star; it's way too close for that. But the planet gives away its presence by the hot and cool faces. We expect to see the hot face when the planet is on the far side of the star from us: it's then we see the lit, hot face of the planet pointed toward us (the orbit is tilted enough that the star doesn't get in the way). That hot face gives off infrared light, which adds a tiny bit of infrared light to the total we see from the system. 2.3 days later, the back side of the planet is presented to us. It's cooler, gives off less infrared, and we see a dip in that light.

By measuring the amount of light, and when we see it, we can infer quite a bit about the planet, like how hot it really is. But astronomers got a surprise...

This graph shows that surprise. It plots the brightness of the system over time, where time is measured in units of the orbital period of the planet. 0 is when the planet's back side is to us, 0.5 is when the lit side is toward us half an orbit later, and 1 is back to the beginning again.

The dark line is what we expect: it should be brightest in the IR when the lit side faces us, and darkest when the unlit side faces us. But that's not what is seen. The dots represent measurements of the amount of IR light coming from the planet, and there is a nice sine wave drawn through them. But look: the wave is out of synch with what we expect. It looks like there is a hot spot on the planet all right, but it's about 80° out of phase with the orbit. It's as if the hot part of the planet is facing to the side of the star instead of right at it!

The diagram on the top showing the planet might make this easier to see. The hot spot on the planet faces us not when the lit side is facing us, but quite a bit before then. When the lit side faces us, the light actually dims, meaning the hot spot is brighter -- hotter -- than the part of the planet where its star is shining right down on it.

What the heck is going on? Why would the hot spot on the planet not be directly under the star, blazing overhead just under 9 million km (5.5 million miles) away? Mind you, the star is hotter than the Sun, too, so that planet is getting cooked.

So, after all that buildup, here's the answer: no one knows. The planet shouldn't be doing what it's doing. It's a mystery. It's almost certainly a gas giant at that mass, so maybe it's rotating so rapidly that it sweeps the hottest air around to a spot 80° away from the star. Small offsets from what's expected have been seen with other planets like this, but 80°? To say that's difficult to understand is an understatement. Maybe there is something happening beneath the clouds, and there's a vast convective cell of upwelling hot gas from deep inside the planet. That's not easy to support either.

Again, the bottom line is, this is a perplexing event. It'll probably keep theorists and atmospheric modelers at their computers for some time to come. Scientists love a challenge, especially one from out of the blue, so to speak.

You may think this is all well and good, observations of a distant object that's not even a dot in the sky, but just dots in graph. But think again. Live in the northern hemisphere? Go outside on the next clear night. This time of year, face east a little after sunset, when the sky is dark. You'll see a giant square in the sky, sitting on one of its vertices like a baseball diamond. That's Pegasus. Streaming off the star on the left side of that square (think of it as third base) is a parallel curved set of lines made up of three stars each. That's Andromeda.

Ups_and_mapIf you look carefully -- binoculars would help -- you'll see a star between the two curves. That's Upsilon Andromedae. It's a star, 44 light years away, but a real star, and orbiting it tightly is a massive Saturn-sized planet, whipping around just above the fires of the star below. That planet too is real, it's a world, and it's doing something weird, something unexpected, something at the moment we can't explain.

We'll figure it out eventually -- we humans are pretty good at that when we set our minds to it. But never forget, that world is real. We can't go there, we can't see it, but it betrays its existence to those who know how to detect it. And the more we learn about this hidden world and the odd things it does, the more we can understand about our own world laid out before us.

That's what science does. And it does it very, very well.

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