Sunset on an alien world

By Phil Plait | January 9, 2012 7:00 am

What would sunset look like if you were on the planet HD209458b, a gas giant orbiting a star 150 light years away? According to exoplanetary scientist Frédéric Pont, it looks like this:

Isn’t that pretty? And there’s quite a bit of science in that, too.

First things first: HD209458 is a star pretty similar to our Sun. It was one of the first stars determined to have a planet orbiting it (way back in 1999) — the aforementioned HD209458b, nicknamed Osiris — and it turns out the planet’s orbit is so close to edge-on as seen from Earth that we see that planet passing directly in between us and that star once per orbit. When the planet transits that star the amount of light we see dips a little bit. From that we can get the period of the orbit and the size of the planet (a bigger planet blocks more light).

But we can get more, too. There’s a camera on board Hubble called the Space Telescope Imaging Spectrograph, or STIS. It can take the light from an object and break it up into thousands of separate narrowly sliced colors, called a spectrum. By analyzing that spectrum we can find out an astonishing amount of things about astronomical objects: their temperature, rotation, even their composition!

Shortly after HD209458b was discovered to be a transiting exoplanet, STIS was pointed at the star. The camera took hundreds of very short exposures during a transit in the hope of being able to detect the atmosphere of the planet. Osiris was known to be massive, about 70% as massive as Jupiter, so it most likely has a thick atmosphere. It also orbits so close to its parent star — 6.7 million km (4 million miles), much closer than Mercury orbits the Sun — that the heat from the star puffs the atmosphere up, making it easier to see.

In fact, the spectra did reveal the presence of an atmosphere; the first time the atmosphere of an alien planet was ever observed. Different elements and molecules absorb light at different colors, so in the spectrum there are dark spots where the planet’s air absorbs the light from the star behind it during a transit, and how dark that spot gets tells you how much light is absorbed.

It’s this information Prof. Pont used to create the image above (inspired by investigation and an animation done by Alain Lecavelier des Etangs). By knowing the color of the star itself, and using the way the planet’s atmosphere absorbs light, he created this image of the star using sophisticated computer modeling. Read More

Brian Marsden, 1937 – 2010

By Phil Plait | November 18, 2010 8:51 pm

I’m sad to say that the eminent astronomer, and my friend, Brian Marsden died today.

This one is tough. I knew Brian; not terribly well, but we were friends. He was the Director Emeritus of the Harvard Smithsonian Minor Planet Center, having run the asteroid and comet clearing house for many years. Anyone from anywhere in the world who discovered a minor body sent the note and data to him. He may not be a household name, but every planetary astronomer on Earth, and many amateurs as well, knew him by name.

You can read his obituary online, and I imagine a lot of bloggers will be writing stories about him — he was quite a character. Let me tell you one of mine.

Back in 1997 or so, I was working on STIS, a Hubble camera that had just been lofted into space and installed into the observatory. We got a lot of pictures from the camera, of course. We had software which automatically grabbed all the new data overnight, and first thing every morning I’d go through them to see if there was anything interesting.

One day, I found a little streak in an image. Ha! I knew right away it was an asteroid. I checked more images, and sure enough we had a total of nine pictures of it! I worked with my friend Eliot Malumuth for a couple of hours on it; the streak it made was curved, which we were able to attribute to the changing perspective of Hubble as it orbited the Earth. But we were able to tease out a rough distance from that — smack in the middle of the main asteroid belt between Mars and Jupiter — and then calculate its size; roughly a kilometer (0.6 miles) across.

Excitedly, I called Brian. I had chatted with him once before, when the asteroid 1997 XF11 was thought to be a potential danger, and was already thinking of names for the rock since I would be granted discovery status. I told him about the asteroid, and could hear him "yes"ing and "mmmkay"ing over the phone. I asked if it would be possible to get followup observations, since we couldn’t get a good orbit over such a short period of time.

That’s when he asked me a simple question. "How bright is it?"

I had already measured that. "24th mag," I promptly replied.

What I heard next over the phone was a surprise: laughter.

And before he could say anything, I knew why he was laughing. 24th magnitude is faint. The faintest star you can see with your eye is a million times brighter than my asteroid. There wasn’t much of a chance a ground-based telescope would be able to follow up, or at least, it wasn’t worth the time of the size of telescope we’d need.

Feeling foolish, I thanked Brian for his time, and we both got a good laugh out of it. That still makes me smile. And no, I never heard of any followup of that asteroid.

One other thing. When we were filming the first episode of Bad Universe, we went to the Minor Planet Center and interviewed Brian. He and I sat on the observer’s chair at the venerable Harvard 15″ telescope, chatting on camera about near-Earth asteroids, the Minor Planet Center’s role, Brian’s role, and other topics. We also ate lunch together with the film crew, where he told us about the history of the observatory. Due to the vagaries of making a TV show, the segment with him didn’t make it to air, but it was a lot of fun to get to hang with him, even if only for a couple of hours or so.

Astronomy won’t be the same without him.


CATEGORIZED UNDER: Astronomy, Bad Universe, Piece of mind

Happy 20th anniversary, Hubble!

By Phil Plait | April 23, 2010 7:00 am

Tomorrow marks the 20th anniversary of the launch of the Hubble Space Telescope. I spent ten years of my life working on that magnificent machine, from using observations of a supernova for my PhD, all the way to helping test, calibrate, and eventually use STIS, a camera put on Hubble in 1997.

Last year, I published Ten Things You Don’t Know About Hubble, and I don’t think I can really add much to it here. I also have a lot of new readers since then, so I’ll simply repost it now as my tip o’ the dew shield to the world’s most famous observatory.


On April 24, 1990, the Space Shuttle Discovery roared into space, carrying on board a revolution: The Hubble Space Telescope. It was the largest and most sensitive optical-light telescope ever launched into space, and while it suffered initially from a focusing problem, it would soon return some of the most amazing and beautiful astronomical images anyone had ever seen.

Hubble was designed to be periodically upgraded, and even as I write this, astronauts are in the Space Shuttle Atlantis installing two new cameras, fixing two others, and replacing a whole slew of Hubble’s parts. This is the last planned mission, ever, to service the venerable ‘scope, so what better time to talk about it?

Plus, it’s arguably the world’s most famous telescope (it’s probably the only one people know by name), and yet I suspect that there are lots of things about it that might surprise you. So I present to you Ten Things You Don’t Know About the Hubble Space Telescope, part of my Ten Things series. I know, my readers are smart, savvy, exceptionally good-looking, and well-versed in things astronomical. Whenever I do a Ten Things post some goofball always claims they knew all ten. But I am extremely close to being 100% positive that no one who reads this blog will know all ten things here (unless they’ve used Hubble themselves). I have one or two big surprises in this one, including some of my own personal interactions with the great observatory!

Ten Things You Don’t Know About Hubble


CATEGORIZED UNDER: 10 Things, Astronomy, NASA, Pretty pictures

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!


See More

Collapse bottom bar