[I'm trying to catch up with all the news that's been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Part 1 was posted earlier.]

Astronomers have found one of the most interesting exoplanets yet: one with a very extended ring system!

[That's an artist's impression of the system; click to encronosate.]

The planet was discovered with the SuperWASP (Wide Angle Search for Planets) telescopes — a UK project that employs low-magnification but very sensitive cameras which can observe large areas of the sky at the same time. It orbits a young star called 1SWASP J140747.93-394542.6, which is 420 light years away. The star’s youth — 16 million years — indicates that the rings are probably the leftover remnants from when the planet formed.

The planet and its rings were discovered using the transit method: looking for small dips in starlight as a planet passes directly between us and the star. This is how the vast majority of exoplanets are found. Usually, when you graph the brightness of the star over time, the dip in the plot as the planet transits the star starts suddenly, drops to some minimum, then jumps back up (see here for example). The whole thing is usually over in a matter of hours at most.

But this planet took nearly two months to transit the star! And the dip was weird: there were multiple times the star dimmed then got brighter again, at one point having 95% of its light blocked. Even though the planet wasn’t seen directly, the most obvious explanation is a ring system similar to Saturn’s (though much larger), blocking the light. It must have gaps in the rings, like Saturn’s do, to explain the starlight jumping up again over time. Overall, four rings were detected, and they stretch tens of millions kilometers in diameter!

Saturn’s ring are only about 300,000 km across, so clearly this planet must be much more massive than Saturn, and the rings denser. It may be a little unfair to compare it to Saturn at all; it’s more like a super-Jupiter still surrounded by primordial debris. Unfortunately, we don’t know how massive the planet itself is; you need Doppler data for that and none has been taken yet. The astronomers who discovered this system, of course, are looking into obtaining Doppler data. It’s even possible the object is so large it’s actually a brown dwarf and not a planet.

Perhaps most intriguing about all this are those gaps in the rings. The easiest way to explain them is that there are objects there, moons, sweeping out the material in the rings. Saturn’s rings have gaps for this reason. In fact, there are hundreds of gaps in Saturn’s rings! These are caused by resonances: if a ring particle orbits twice for every one time a moon orbits, for example, the moon’s gravity tugs on it every time it swings by, pulling it into a different orbit. Over time, all the particles in that orbit are gone, leaving behind a gap.

If the planet itself is big, how big are those moons? Could one be Earth-sized? It’s an idea that’s been around awhile, but none has ever been seen… yet. All these super-Jupiters being found have a lot of gravity, and it’s possible they have big moons. We’re also getting better at detecting smaller objects, so it wouldn’t surprise me if that announcement is made sometime relatively soon, too!

I’ll note that the idea of looking for rings and moons is more than an idea: the Hubble observations of the star HD 209458 I mentioned the other day were taken to look for moons and rings around that planet! None were seen, but astronomers will keep trying. There are a lot of planets out there, and one thing we’ve learned is that variety is the spice of nature.

Image credit: Michael Osadciw/University of Rochester

There’s been so much exoplanet news this week! I was in Texas the past couple of days giving a bunch of talks, so I’m trying to catch up. All the exoplanet news is way cool, but too much for one post, so I’ve split them up. I’ll post the other parts shortly.

Part 1: A trio of hot little rocks

First up? The three smallest exoplanets found so far. I usually don’t like to write about incremental discoveries, but this one is truly cool: all three orbit the same star, and all three are smaller than Earth! Any one of these would be a record breaker, but to find all three at once, in the same place? Amazing.

They orbit the star KOI-961 (short for Kepler Object of Interest), and were observed by the Kepler Observatory (details on how that all works can be found here). They all orbit the star extremely close in: the farthest one is a mere 2.3 million km (1.5 million miles) from the star! They’re so close they all take less than two days to circle it once. And even though the star is a red dwarf, and therefore relatively cool, they are so close to it that they probably resemble airless, heat-blasted Mercury more than Earth. They are almost certainly rocky/metallic bodies, since they are so small: 0.78, 0.73 and 0.57 times the diameter of the Earth. Although we’ve been surprised before, it’s hard to imagine anything that small could hold onto much atmosphere when they are so hot.

Funny, too: the star is tiny, only a bit bigger than Jupiter. And the planets are so close in the KOI-961 system looks more like Jupiter and its moons than our own solar system! The artwork above drives that point home. Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons. [Edited to add: Note that the distances are not to scale!]

Why is this news important? Well first, it adds more weight to the idea that planets smaller than Earth exist and can be found around other stars. Second, it shows that red dwarf stars can form and hold onto planets… which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars! So finding multiple planets around one means, once again, planets are almost certainly common in the galaxy.

And third, it just shows once again that the Universe is a surprising place. This mini-solar system proves that nature is diverse, and will fill any niche it can. It also implies, very strongly, that we need to broaden our concepts of how solar systems form, what they look like, and how they behave.

Image credit: NASA/JPL-Caltech

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. (more…)

[NOTE: I have been informed that this is NOT the first planet seen in the habitable zone of another star, but the first seen by Kepler, and moreover the first that is not a gas giant. Rather than try to correct the text below using strikethroughs, which would be confusing, I simply edited the text. I hope that's clear!]

This is pretty big news: the space-based Kepler observatory has confirmed it has found its first planet in the habitable zone of a star like the Sun! Not only that, the planet may well be similar to Earth, though that’s not clear yet.

The planet, called Kepler-22b, is about 600 light years away. The star it orbits, called simply Kepler-22, is a bit lower mass and cooler than the Sun. The planet takes about 290 days to circle the star once, and as soon as I saw that number I let out a little "yip" of surprise — that number’s perfect! Why?

Because that puts the planet inside of that star’s habitable zone, the distance where, given certain planetary conditions, liquid water can exist. It may be that life can arise where there’s no water, but we know life on Earth needs water, so if we’re looking for habitable planets it makes sense to look for the possibility of water there.

The planet is closer to its star than Earth is to the Sun — that’s why its year is shorter — but the star is cooler, compensating for that. That makes this the best candidate yet for Earth-like conditions. But is the planet like our own world?

That’s hard to say.

(more…)

Two new planets orbiting other stars have recently been discovered using NASA’s orbiting Kepler telescope. And while every new planet discovery is pretty amazing, normally two more add to the hundreds already confirmed wouldn’t really be newsworthy. However, these two weren’t discovered by professional astronomers! They were found by members of the Planet Hunters "citizen scientists" team; regular folks who have volunteered to sift through data returned by the observatory in hopes of finding far-flung worlds.

One of the planets found orbits its star with a period of just under 10 days, and the other orbits a second star in just under 50 days. Both are much more massive than Earth; the first is 2.65 times and the second over 8 times our diameter. The relatively lower mass means the first one might be rocky (as opposed to a gas giant) but the short period means it’s hot, far hotter than Earth.

Both planets transit their stars as seen from Earth. In other words, they pass directly in front of their stars from our point of view, blocking the light a wee bit. This drop can be measured, and the planet detected. By knowing how big the star is (a dwarf, a giant, whatever) the period of the planet can be found, and the size of the planet can be determined by how much light is blocked, too.

The Kepler observatory is staring at about 100,000 stars all the time to look for these mini-eclipses, and astronomers use a fleet of software to automatically tag suspicious changes in starlight. But it’s pretty hard to look through all the potential planet data, and that’s why Planet Hunters was set up: let people go through the data themselves, using their keen eyes and powerful brains to look for anything that might be a planet.

And it worked! The two planets discovered were just announced in a paper led by the Kepler team (PDF). Here’s a plot showing one of the transits:

(more…)

On Sunday I posted about NASA’s twin STEREO spacecraft, which are now 90° ahead and behind the Earth in its orbit. From their vantage point, over 200 million kilometers away, they can together see the entire far side of the Sun and beam the images back to Earth, providing us with real time data impossible to get from home.

While I was going through old blog posts to look at entries I had written about STEREO, I found one showing some STEREO data that I thought was worth showing everyone again. Putting it in Sunday’s article would’ve made it too long, so here it is on its own.

Let me interject a personal note first. I was at NASA’s Goddard Space Flight Center working on Hubble when STEREO was first being put together as a mission. I remember thinking how cool it would be to see the entire Sun at the same time, and what it would mean to my friends over at the heliospheric physics section. I have a decent imagination, but still there was no way I could’ve ever foreseen some of the things STEREO has brought us — Nature is always more clever than any one of us. And my favorite of all of them, sent back while the two spacecraft were still relatively near the Earth, is this incredible animation showing something that can never be seen from Earth: the tiny disk of the Moon transiting the Sun:

From Earth, that would be a solar eclipse, where the black disk of the Moon would look the same size as the bright Sun. But from well over a million kilometers away — the distance STEREO B was when it took these images — the Moon is smaller, providing this eerie and beautiful view that is a stunning reminder that humans are a spacefaring species, and the views we get from there expand our world.

Yesterday I posted a beautiful image of the Moon transiting the Sun’s disk as seen from the Solar Dynamics Observatory. Well, today NASA put up HD video of the event!

[It may take a minute to load, it's a big file.]

Wow! You can see the Sun’s rotation over the four hour interval these pictures were taken, and then the Moon flashes by. The Moon is dark because it’s between us and the Sun, so were seeing the unlit side (I have a diagram showing an approximation of the geometry of this event on my other page about it).

Moreover, scientists can use images of the sharp-edged Moon against the Sun’s disk to check on the optics of the observatory. That information can be used to sharpen the images even more. So this is fantastic, fantastically beautiful, and fantastically useful, too.

Tip o’ the sunglasses to NASAGoddard.