David Kipping is an astronomer at the Harvard-Smithsonian Center for Astrophysics, one of the most prestigious astronomical research institutions on Earth. He studies exoplanets: planets orbiting other stars. Specifically, he’s interested if they have moons. Why? Because big, massive planets are the easiest to find, but even if they orbit their parent star at the right distance to have liquid water, they’re too big to harbor life as we know it. But it’s possible some of these giants might have moons that could! After all, both Saturn and Jupiter have moons we know are made of water (mostly ice, but also some have liquid water).
The problem is detecting them. It’s a difficult, complicated problem given the data we have, but Dr. Kipping thinks he may have a way to tease them out of the observations. The problem is simply computational time, and that means a fast, dedicated computer. Getting funds for that is hard, so he’s set up a campaign to raise money on petridish.org — which is like KickStarter but for science — called Help Us Find the First Exomoon. He’s hoping to raise $10,000 to get the computer he needs.
Here’s his pitch:
Not bad. This is a worthy project, and one I think is very cool as well. I know some folks who study exoplanets, and they’re very excited about looking for moons. It pushes our technology to the limit, but it certainly can be done. And you can help make it happen!
And I have to chuckle; he used images by my friend Dan Durda, whose artwork has graced my blog many times (the painting at the top is one of Dan’s) and some time lapse footage from TimeScapes, which I’ve featured here before as well. Small world. Or worlds.
Tip o’ the Doppler wobble to geologist Matthias M. M. Meier.
As we find more and more planets orbiting other stars, we keep finding ones that are weirder and weirder. Enter GJ 1214b: while much more massive than the Earth, it’s apparently mostly water!
[Click to enhydronate this artists's illustration.]
The planet — orbiting the star GJ 1214 at 40 light years from Earth — was actually discovered in 2009 by the MEarth project, which is looking for Earth-like planets around, cool, dim red dwarf stars. This is fertile ground for the search: these stars are extremely common, making up something like 80% of the stars in the sky. Not only that, but because they are cool, a planet at the right temperature to have liquid water on the surface would have to be close to the star. That means its period is shorter, making it easier to find (you don’t have to wait a long time for the effects of its orbit on the star to be seen).
In this case, though, it’s not terribly Earthy! First off, it’s massive, weighing in at
2.7 6.5 times our own planet’s mass. It’s also orbiting the star at a distance of a mere 2 million kilometers, giving it a temperature of something like 230° Celsius (450° F): hot enough to roast a chicken.
But it apparently has something the Earth does: water, and lots of it. From our viewpoint, the planet passes directly in front of its star once every 38 hour orbit (this is called a transit). When it does, it blocks the star’s light a little bit (which is how it was discovered). But the planet also has a thick atmosphere, and when it passes in front of the star, that atmosphere absorbs some of the starlight. As it happens, different things in the atmosphere absorb light differently. Water vapor, for example, has a different impact on a spectrum taken of the star’s light than, say, carbon dioxide.
So by breaking the light up into lots of colors and carefully measuring it, it’s possible to figure out what’s in the planet’s atmosphere. Earlier observations could tell that something was absorbing starlight, but they couldn’t tell what. New Hubble observations (PDF) indicate that the best fit to the observations is: water. Haze, for example, absorbs more visible light than infrared, but that’s not what was seen. The spectrum matches the way water absorbs light best, and in fact indicates the atmosphere may be as much as 50% water by mass!
Given the planet’s size of about 35,000 km (22,000 miles), its density is quite low: about 2 grams per cubic centimeter. Compare that to Earth’s density of 5.5 grams per cc! A rocky world more massive than Earth would most likely be denser than 2 grams/cc, so that’s consistent with this planet having lots of water (which has a density of 1 gram/cc).
The scientists involved indicate this planet would be really weird. There may be exotic forms of water there, due to the high temperatures and pressure deep in the planet’s atmosphere. Most likely the planet formed farther out from the star and migrated inward, a phenomenon that is apparently very common in planetary systems. I’ll note other planets like this have been found, too, but not with such a high-precision spectrum and therefore such certainty.
I know that our solar system is pretty weird; we have all manners of strange things floating around in it. But there’s nothing like seeing something so weird it makes us look positively normal by comparison. Sometimes we really do need a swift kick in the planets.
[P.S. My hearty congrats to Zach Berta, the lead author on the Hubble observations. I got to interview him for an Episode 2 of "Bad Universe", and he was very welcoming and fun to hang out with (and got an IMDB credit out of it). We talked quite a bit about GJ 1214b for the interview, and I'm glad to see this planet and its discoverer get some press!]
Image credit: NASA, ESA, and D. Aguilar (Harvard-Smithsonian Center for Astrophysics)
- Exoplanet in a triple star system smack dab in the habitable zone
- Motherlode of potential planets found: more than 1200 alien worlds!
- Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!
- Super Venus steampunk planet!
In the race to find the weirdest planet orbiting another star, we may have a front runner: GJ 667Cc, a super-Earth orbiting one star in a triple system that’s actually relatively closeby. And oh yeah: it just so happens to be in just the right spot to be potentially inhabitable!
Of course, I have some caveats, so don’t get too excited. But this is a weird and pretty cool one!
GJ 667 is a triple star system that’s right in our back yard as these things go: it’s only about 22 light years away, making it one of the closest star systems in the sky. It’s composed of two stars a bit smaller and cooler than the Sun which orbit each other closely, and a third, smaller star orbiting the pair about 35 billion km (20 billion miles) out. Stars in multiple systems get capital letters to distinguish them, so the two in the binary are GJ 667 A and B, and the third one is GJ 667C.
That third star is the interesting one. It’s a cool, red M dwarf with about a third the diameter of the Sun. Fainter, too: it only puts out about 1% of the light the Sun does. It’s been studied for years to look for planets around it, and while there have been some signs found, this new research is the first solid detection of planets that’s been published.
They used the Doppler method (sometimes called the Reflexive Velocity method): as planets orbit a star, their gravity tugs on it. We usually can’t see this motion directly, but a spectrum can reveal a Doppler shift, similar to the change in pitch you hear when a car or train goes by. If the spectrum has a high enough resolution, and the analysis very carefully done, there’s a lot you can tell by measuring it. You can get the planet’s mass, its period, and even the shape of its orbit.
In this case, the spectrum reveals GJ 667C may have four planets! Two very strong signals pop up with periods of 7 and 28 days, a third one at 75 days, and a possible trending shift in the spectrum that may point to a planet orbiting in a very roughly 20 year period.
It’s that second planet, GJ 667C with a 28 day orbit that’s so interesting. Its mass is at least 4.5 times that of the Earth, so it’s hefty. A 28 day orbit puts it pretty close to the parent star — about 7 million kilometers, or less than 5 million miles (Mercury is 57 million km from the Sun, by comparison). But remember, GJ 667C is a very dim bulb, so being that close means that the planet is actually right in the middle of the star’s habitable zone! The HZ is the distance where liquid water could exist on a planet — it depends on the size and temperature of a star, and also on the planet’s characteristics. A cloudy planet can hold heat better through the greenhouse effect, so it can be farther from the star and still be warm, for example.
[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. Here's Part 1, Part 2, and Part 3.]
In September, astronomers announced the discovery of a planet (Kepler-16b) that orbited not one but two stars. The stars orbit each other (in what’s called a binary system) and the planet circles both. This was the first such planet found doing this (out of hundreds of planets orbiting single stars discovered), which opened up the question: how rare is this kind of system? Is Kepler-16b one of a kind?
The answer appears to be no: two more such systems have just been announced! Dubbed Kepler-34b and Kepler-35b, both are gas giants, similar in size to Saturn.
The planet Kepler-34b orbits two Sun-like stars once every 289 days. The two stars (Kepler-34A and Kepler-34B; note the capital letter denoting a star versus the lower case letter denoting a planet — which technically should be called Kepler-34(AB)b, but at some point I have to draw the line and simplify) orbit each other every 28 days. The planet Kepler35-b orbits a pair of somewhat lower-mass stars every 131 days (the stars orbit each other every 21 days).
Note that in both cases, the planets orbit their stars at distances much larger than the distances between the two stars themselves. That’s not surprising to me. From far away, a circumbinary planet (literally, "around two stars") feels the combined gravity of the two stars more than either individual star, much like distant headlights on the highway look like a single light. When you’re close, the two lights resolve themselves. Same thing with a planet; if it orbits much closer in the gravity field is a bit more distorted by the individual stars. Too close, and the orbit becomes unstable and the planet can be ejected from the system entirely! But it looks like both Kepler-34b and 35b have nice, stable orbits.
Binary stars are very common in the Milky Way: roughly half of all stars are binary, and now we know that at least three such systems have circumbinary planets. And we’ve only just started looking! Mind you, these planets were found using the transit method, so the orbits have to align just right from our viewpoint or else we don’t see them transit. For every one transiting system we find there are many more that exist but don’t transit, so we don’t see them. But they’re out there.
I suspect that the fraction of binary stars with planets is probably lower than for single stars, since planets forming (or moving) closer in to the binary center will get ejected. But still, even with a lower fraction we’re still talking about a pool of hundreds of billions of stars, so it’s likely that there are millions of circumbinary planets out there: millions of Tatooines!
And hmmmm. Kepler 34 and 35 are 4900 and 5400 light years away, respectively, making them among the more far-flung planetary systems seen. You might say that if there’s a bright center to the Universe, they’re the planets that it’s farthest from.
I’ve always dreamed of standing on a hill and watching twin suns set in the west. Sadly, the wind won’t blow through my hair like it did Luke Skywalker’s, but that would be a small price to pay. What a view that would be!
[UPDATE: Wait a sec! Right after posting, I realized: the two planets are both gas giants, but far enough from their stars that big, terrestrial moons might be possible. So imagine that: a binary sunset with a gigantic planet looming in the sky as well! That would be incredible.]
Image credit: Lynette Cook and SDSU
A very interesting set of observations has resulted in a conclusion that is somehow, paradoxically, both expected and startling: there are hundreds of billions of planets in our galaxy alone!
It’s expected because all the research being done for the past few years has been zeroing in on how many stars have planets, and it’s looking more and more like they’re very common. I’ll get into that in a sec. But it’s also startling, because HOLY COW THERE MAY BE HUNDREDS OF BILLIONS OF PLANETS IN OUR GALAXY ALONE!
Ahem. OK. So what’s going on here?
The new result comes from what’s called microlensing. The gravity of a star or planet can bend the light coming from an even more distant star, briefly magnifying it. The way the star light gets brighter over time can reveal the mass of the object doing the magnifying — the "lens", as it were. If a star passes in front of another star, you get a rise and then fall in the brightness, but if a planet is orbiting that nearer star, you get a second, smaller bump as well.
This kind of event takes an extraordinarily precise alignment, so they’re extremely rare. To compensate, you need to look at a lot of stars. So astronomers did: a survey using two telescopes covered several million stars every night, looking for the tell-tale bump(s). Over the course of six years, they found three — yes, only three — planets orbiting other stars acting like wee distant lenses. But that number is actually pretty good: when combined with previous surveys, and also taking into account how many lenses they didn’t see (which is important, statistically), they can extrapolate with some confidence about the numbers and types of exoplanets out there.
Their most basic result, and the one causing the stir, is that they find that there are likely hundreds of billions of planets orbiting other stars in our galaxy alone. Given that there are a few hundred billion stars in the Milky Way, this means on average there are about one or two planets per star in our galaxy! Now, let me be clear: this is an average. I’ve seen reports saying every star in our galaxy has a planet, and that’s not necessarily the case. You could have one star, say, with ten planets, and then nine with none and get the same results here.
The results get even more interesting when you break them down by planet type. Read More
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
- Kepler finds a mini solar system!
- Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!
- No, it’s *not* the smallest exoplanet found!
- A boiling superEarth joins the exoplanet roster
[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.
A collaboration between space- and ground-based telescopes has added a new world to the growing list of exoplanets: Kepler-21b, a planet bigger and more massive than Earth. It’s far smaller than Jupiter, though, putting it firmly in the "small, rocky planet" category. Not that it’s Earth-like: it orbits its star in just under 3 days, making it hot enough to have pools of molten iron on its surface!
Now, I don’t generally write about every new alien planet discovered — with over a thousand of them and counting, it would be all I ever do! — but this one interested me. For one thing, it’s not all that much bigger than Earth; it’s about 1.6 times our diameter. The size was able to be found because the planet transits its star: it passes directly between the star and us, blocking the star’s light a wee bit. The amount of light blocked depends on the size of the planet itself, so by carefully measuring that dip in brightness the planet’s size can be determined.
And did I say a wee bit? I mean a really wee bit! Here is a graph showing the planet’s effect on the starlight:
The vertical axis is the amount of light we see from the star, and the horizontal axis is time. You can see how the light drops a bit when the planet blocks the star. But look at the scale! The planet blocks a mere 0.005% of the star’s light! That’s an incredibly sensitive detection, and incredibly difficult to detect. Stars have all sorts of ways of varying their light output, from sunspots to intrinsic pulsing. All those effects had to be removed from the observations to find this weak leftover signal.
But that’s the power of multiple observatories. The star was observed by the orbiting Kepler observatory, designed to look for such planets transiting their stars. It was followed up by the ground-based Mayall and WIYN telescopes at Arizona’s Kitt Peak National Observatory for confirmation, and in total the planet was watched for over 15 months to determine its characteristics.
Even better, these combined observations tell us the mass of the planet itself. As it circles its star every 2.8 days, its gravity pulls on the star, subtly changing the spectrum of the star’s light. The more mass a planet has, the more gravity, and so the more it pulls on the star, and the bigger the effect on the spectrum.
In this case, the planet has a mass of no more than 10 times that of Earth, and is probably less. Read More
This is truly astonishing: an “amateur” astronomer in New Zealand, Rolf Olsen, has for the first time actually been able to get a direct photograph of the disk of swirling material forming a planet around a nearby star!
OK, first, the picture:
This may not look like much at first glance, but that’s often true of amazing pictures. When you realize what you’re actually seeing…
This is a picture of Beta Pictoris (or just β Pic to those in the know), a young star just over 60 light years away. The light from the star itself has been subtracted away (more on that in a sec), and the two big crosshair streaks of light are called diffraction spikes — they’re caused by light inside the telescope and aren’t real. But the fuzz you see above and below the star is real, part of the disk of material forming planets right before our eyes! The dashed line was added by Rolf to show the orientation of the disk.
In the 1980s, infrared images of β Pic revealed that it’s surrounded by a flat dust disk almost exactly edge-on to us. We see that disk as a broad line crossing the star itself, like in the false-color image here from the Las Campanas observatory.
β Pic became a very popular object, with many telescopes pointed at it to try to determine the nature of this material. This was the first time we had ever directly seen the disk of protoplanetary material. We now know that not only is that disk actively forming planets, there is a planet orbiting the star inside that disk, and we’ve even seen it move!
In 1998, the Hubble Space Telescope was pointed at HR 8799, in hopes of seeing any potential planets that might be orbiting that nearby, Sun-like star. None were found… but in 2008 images using the Gemini telescope found several planets orbiting HR 8799. In fact, four planets were discovered there!
So why weren’t they seen in the Hubble data? The star was too bright, and software techniques in 1998 couldn’t sufficiently remove the star’s light to reveal the much fainter planets. But things have changed in 13 years, and astronomers went back to the old HST data, using newly-developed methods to clean the images. And lo, they saw three of the four planets!
[Click to exoplanetate.]
On the left is the image of the star as seen by Hubble’s infrared camera NICMOS. On the right, the star’s light has been subtracted, and the three planets (HR 8799 c, d, and b, from left to right) can be clearly spotted.
The very cool thing is that this older image gives us a much longer baseline over which we can see the planets. Why is that cool? Because the planets have moved over those 10 years between images! By comparing the old image and the one from 2008, the motion of the planets as they orbit the star can be directly seen and measured. Calculations based on that observation show the planets d, c, and b take about 100, 200, and 400 years to circle HR 8799 once, respectively. Given more time, we’d've figured that out anyway, but getting this image from Hubble is like getting an extra 10 years for free. So these planets join one around the star Beta Pic and another around Fomalhaut as being directly seen to move over time. Pretty amazing.