Well now, this is an interesting discovery: astronomers have found what looks like a "super-Earth" – a planet more massive than Earth but still smaller than a gas giant – orbiting a nearby star at the right distance to have liquid water on it! Given that, it might – might – be Earthlike.
This is pretty cool news. We’ve found planets like this before, but not very many! And it gets niftier: the planet has at least five siblings, all of which orbit its star closer than it does.
Now let me be clear: this is a planet candidate; it has not yet been confirmed. Reading the journal paper (PDF), though, the data look pretty good. It may yet turn out not to be real, but for the purpose of this blog post I’ll just put this caveat here, call it a planet from here on out, and fairly warned be ye, says I.
The star is called HD 40307, and it’s a bit over 40 light years away (pretty close in galactic standards, but I wouldn’t want to walk there). It’s a K2.5 dwarf, which means it’s cooler, dimmer, and smaller than the Sun, but not by much. In other words, it’s reasonably Sun-like. By coincidence, it appears ot be about the age as the Sun, too: 4.5 billion years. It was observed using HARPS, the High Accuracy Radial Velocity Planet Searcher (I know, it should be HARVPS, but that’s harvd to pronounce). This is an extremely sensitive instrument that looks for changes in the starlight as a planet (or planets) orbits a star. The gravity of the star causes the planet to orbit it, but the planet has gravity too. As it circles the star, the star makes a littler circle too (I like to think of it as two kids, one bigger than the other, clasping hands and swinging each other around; the lighter kid makes a big circle and the bigger kid makes a smaller circle). As the star makes its circle, half the time it’s approaching us and half the time it’s receding. This means its light is Doppler shifted, the same effect that makes a motorcycle engine drop in pitch as it passes you.
Massive planets tug on their star harder, so they’re easier to find this way. Also, a planet closer in has a shorter orbit, so you don’t have to look as long to find it. But in the end, by measuring just how the star is Doppler shifted, you can get the mass and orbital period of the planet. Or planets.
In this case, HD 40307 was originally observed a little while back by HARPS, and three planets were found. But the data are public, so a team of astronomers grabbed it and used a more sensitive method to extract any planetary signatures from the data. They found the three previously-seen planets easily enough, but also found three more! One of them is from a planet that has (at least) seven times the mass of the Earth, and orbits with a 198 day period. Called HD 40307g (planets are named after their host star, with a lower case letter after starting with b), it’s in the "super-Earth" range: more massive than Earth, but less than, say Neptune (which is 17 times our mass).
We don’t know how big the planet is, unfortunately. It might be dense and only a little bigger than Earth, or it could be big and puffy. But if its density and size are just so, it could easily have about the same surface gravity as Earth – that is, if you stood on it, you’d weight the same as you do now!
But the very interesting thing is that it orbits the star at a distance of about 90 million kilometers (55 million miles) – closer to its star than is is to the Sun… but that’s good! The star is fainter and cooler than the Sun, remember. In fact, at this distance, the planet is right in the star’s "habitable zone", where the temperature is about right for liquid water to exist!
That’s exciting because of the prospect for life. Now, whenever I mention this I hear from people who get all huffy and say that we don’t know you need water for life. That’s true, but look around. Water is common on Earth, and here we are. We don’t know that you need water for life, but we do know that water is abundant and we need it. We don’t know for sure of any other ways for life to form, so it makes sense to look where we understand things best. And that means liquid water.
Here’s a diagram of the system as compared to our own:
Note the scales are a bit different, so that the habitable zones of the Sun and of HD 40307 line up better (remember, HD 40307g is actually closer to its star than Earth is to the Sun – an AU is the distance of the Earth to the Sun, so HD 40307 is about 0.6 AU from its star). What makes me smile is that the new planet is actually better situated in its "Goldilocks Zone" than Earth is! That’s good news, actually: the orbit may be elliptical (the shape can’t be determined from the types of observations made) but still stay entirely in the star’s habitable zone.
And take a look at the system: the other planets all orbit closer to the star! We only have two inside Earth’s orbit in our solar system… but all five of HD 40307′s planets would fit comfortably inside Mercury’s orbit. Amazing.
So this planet – if it checks out as being real – is one of only a few we’ve found in the right location for life as we know it. And some of those we’ve found already are gas giants (though they could have big moons where life could arise). So what this shows us is that the Earth isn’t as out of the ordinary as we may have once thought: nature has lots of ways of putting planets the right distances from their stars for life.
We’re edging closer all the time to finding that big goal: an Earth-sized, Earth-like planet orbiting a Sun-like star at the right distance for life. This planet is a actually a pretty good fit, but we just don’t know enough about it (primarily its size). So I’m still waiting. And given the numbers of stars we’ve observed, and the number of planets we found, as always I have to ask: has Earth II already been observed, and the data just waiting to be uncovered?
Image credits: ESO/M. Kornmesser; Tuomi et al.
- ALPHA CENTAURI HAS A PLANET!
- Kepler confirms first planet found in the habitable zone of a Sun-like star!
- A nearby star may have more planets than we do
- Exoplanet in a triple star system, smack dab in the habitable zone
- Super-Earth exoplanet likely to be a waterworld
First it was there, then it wasn’t, and now it just may be back again: the first exoplanet directly observed orbiting a normal star, Fomalhaut b, has had quite a ride.
[This post has a bit of detail to it, so here's the tl;dr version: new analysis shows an object orbiting the star Fomalhaut may actually be a planet, enveloped in a cloud of dust. We can't for sure it exists, but we can't say it doesn't, either! Earlier claims of it not existing may have been premature. Also, at the bottom of this post is a gallery of direct images of exoplanets.]
First a brief history. In 2008, astronomers revealed huge news: they had successfully taken images of planets orbiting other stars. Up until then, the only evidence we had of exoplanets was indirect, either by their tugging on their stars which affects the starlight, or by having them pass between their stars and us, dimming the starlight.
But, along with Gemini telescope pictures of a family of planets orbiting HR 8799, Fomalhaut b was the first planet ever seen directly, as a spark of light in a picture. Here is that historic shot:
It’s Sauron’s eye! [Click to embiggen.]
The object is labeled. It doesn’t look like much, but the important thing to note is that it moved between 2004 and 2006 (see picture below), and it was definitely in both images taken two years apart. That means it wasn’t some bit of noise or detector error. Moreover, the movement was consistent with what you’d expect from a planet. Not only that but the star Fomalhaut is surrounded by a vast ring of dust – Sauron’s eye – and the inner edge of the ring is sharp. That’s what you would expect if a planet was orbiting inside the ring; its gravity sweeps up the dust on the inside of the ring. Given the brightness, we were looking at an object with a few times Jupiter’s mass, much smaller than a star, so definitely a planet.
All in all, it looked good, and it looked real.
Then, in early 2012, some astronomers threw a Pluto-esque wet blanket on the news. A planet that big should be bright in the infrared. Fomalhaut is a youngish star, only a few hundred million years old. Any planet more massive than Jupiter should still be hot, radiating away the heat of its formation. They looked for it in the infrared, and it wasn’t there.
To make things worse, they found that if you extrapolate the orbit of the supposed planet using its movement, it should cross the ring. That’s bad, because its gravity would disrupt the ring after a few million years tops. The ring is there, so that planet means the planet must not be.
Their conclusion: this object is a clump of dust, a cloud, orbiting the star. That fits the data, and a planet doesn’t. Cue the sad trombone.
But wait! We’re not done!
Well, this is depressing: Fomalhaut b may not exist.
Fomalhaut is one of the brightest stars in the sky, and is only about 25 light years away — that’s close, on a cosmic scale. It’s young, not more than a few hundred million years old, and surrounded by a vast ring of dust, leftover from the formation of the star itself. The ring is about 20 billion km (12 billion miles) in radius, and has a sharp inner edge.
That last bit is important: the easiest way we know to make the inside edge that well-defined is if a planet is orbiting the star just inside the ring. Its gravity would draw in particles, sculpting what would otherwise be a fuzzy boundary into a clean-cut ring. Not only that, but the ring is off-center; again, that’s likely due to the gravitational influence of a planet.
In 2008, astronomers announced they had found that planet: it appeared in two different Hubble Space Telescope images (shown above; click to embiggen) separated by two years. During that time, it had moved a little bit, by just what you’d expect for a planet at that distance from the star. The news came out the same day as other planets were seen around a different star, and I, along with lots of other folks, made it a headline (see the gallery at the bottom of this post showing all the planets we’ve been able to detect directly in images). This was, after all the first direct detection of a planet orbiting a Sun-like star!
Except, maybe not so much. A new paper has come out (PDF) trying to see Fomalhaut b using the Spitzer Space Telescope. Spitzer is sensitive to infrared, where the planet is far brighter.
And what did they see? Nothing.
This image is pretty damning for the existence of Fomalhaut b. It’s the Spitzer infrared observations of the star, with the star’s light carefully removed. On the left is the actual image, and on the right they artificially added a point of light calculated to be equal to what the planet would emit, in the same position the planet should be — that’s what Arrow 1 is pointing at. It should be one of the brightest things in the image (Arrow 2 points to an unrelated bright spot). And while it’s obvious on the right, nothing can be seen on the left, in the real image. In other words, the planet isn’t seen.
Looking over the paper, it’s clear the astronomers were very careful, and did a number of tests. There’s no known way to make a planet as bright as what was seen in the Hubble images yet invisible in the Spitzer images. If the planet were there, they should’ve seen it. Also, a recent study has shown that if the two images show the planet moving, it would be on an orbit that crosses the ring! That seems extremely unlikely, if not outright impossible. A planet that big and massive — more massive than Jupiter — would disrupt the ring in short order if it physically crossed it. That really does make it very, very likely this is not a planet*.
So what is it? It’s probably a clump of dust orbiting the star, reflecting light from the star enough to show up in the Hubble images but not warm enough to show up in the infrared observations.
That’s too bad. If this is true — and it probably is — then that takes away one of the very few planets directly seen in telescopic observations. However, there are still plenty more, and those have been confirmed (again, see the gallery below). And that number will tend to increase as time goes on, even if every now and again it drops by one or two.
Hmph. I once wrote that destroying a planet is hard. Sometimes, all you need to do is try to observe it a different way, and poof! It’s gone.
And now I have to update that gallery, and all my previous pages about it too. Dang science. Always learning more stuff and changing what we thought we knew.
Image credit: Paul Kalas, U C Berkeley; NASA/Spitzer/Markus Janson et al.
* I chatted with an astronomer friend of mine about this, and he agreed with the authors of this new study. "Overall," he wrote me, "it smells like fish.". I couldn’t help myself. I wrote him back: "Of course it does. Fomalhaut is the brightest star in Pisces!"
[Below is a gallery of exoplanets that have been directly imaged using telescopes on ground and in space. Click the thumbnail picture to get a bigger picture and more information, and scroll through the gallery using the left and right arrows.]
[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
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
Astronomers have achieved a big milestone in the search for another Earth: the two smallest confirmed planets ever found orbiting another star… and they’re both about the size of Earth!
|Artist’s illustration of the Kepler-20 planets with Earth and Venus for size comparison.|
The planets are called Kepler-20e and Kepler-20f, and as you can see by the illustration above they are very close to the same size as our home world: 20e is about 11,100 km (6900 miles) in diameter, and 20f about 13,200 km (8200 miles) across. For comparison, Earth has a diameter of 12,760 km (7930 miles). This makes them the smallest confirmed exoplanets seen orbiting another star! The previous record holder was Kepler-10b, which has a diameter about 40% bigger than Earth’s.
To be clear: while these planets are the size of Earth, they are nowhere near Earth-like. The star, Kepler-20, is very much like the Sun, though a bit smaller and cooler (and 950 light years away). However, both planets orbit the star much closer than Earth does; 7.6 million km (4.7 million miles) and 16.6 million km (10.3 million miles), respectively. This is so much closer that both planets must have surface temperatures far hotter than ours, 760°C and 430°C (1400°F and 800°F). Even on the "cooler" planet Kepler-20f, it’s hot enough to melt tin and zinc.
So don’t start packing your bags to visit, even if you could spare a few million years to get there via rocket (950 light years is a bit of a hike). I’ll note that we don’t know the masses of these planets either. I’ll explain that in a moment, but given their sizes it’s expected they’ll have masses similar to Earth’s.
So this is very exciting! For one thing, it shows that Kepler can indeed find planets the size of Earth orbiting distant stars. That right away is fantastic; that’s the main goal of Kepler in the first place.
For another, it shows that our solar system is not entirely unique. We do know of several other stars hosting solar systems of their own, but those planets tend to be very massive; they’re easier for us to find. Since Kepler-20e and f are so close to Earth-sized, this is a big achievement.
And we’re still not done: there are three other planets in the Kepler-20 system! Read More
Astronomers may have, for the first time, directly imaged a planet still in the process of formation, gathering material from a debris disk surrounding its parent star.
First: Holy Haleakala!
Second: note the use of the word "may". It looks to me like it’s real, though.
Third: Oh, you want to see the picture? Well, let me do the honors:
The alleged planet, called LkCa 15b, is the blue spot in the image. The red shows material which is most likely accumulating onto the planet itself, building up its mass. The central star isn’t seen in this image because its light has been blocked out so the fainter material near it can be seen. The star’s position is marked by the star icon.
The image is in the infrared, taken using the monster Keck telescope in Hawaii. What’s shown in red is light at a wavelength of 3.7 microns (roughly five times what the human eye can see) and blue is from 2.1 microns, about three times what we can see. Warm material around the star is best seen at these wavelengths. If this is a planet, it’s at a temperature of about 500 – 1000 K (440° – 1340° F), and has a mass roughly six times that of Jupiter, or about 2000 times the Earth’s mass.
So is it a planet? Read More
If there’s a bright center to the Universe, astronomers have found the planet it’s farthest from. Called Kepler-16b, it’s a Saturn-like world which has the distinction of being the first discovered to orbit both Sun-like stars in a binary system.
OK, Star Wars references aside, this is pretty cool. Most of the planets being found around other stars are orbiting single stars. A very few — like a possible planet orbiting Gamma Cephei — orbit one of the stars in a binary system, and some (like NN Serpentis b and c) orbit both stars, but one of them is a dead star like a white dwarf or a neutron star.
Unlike those, Kepler-16 is a binary where both stars, though dinky, are bona-fide stars like the Sun, and the planet orbits both. Actually, how it was found is pretty nifty. The orbiting Kepler observatory is designed to stare at over 100,000 stars and detect the tell-tale drop in light when a planet transits (that is, from our point of view passes directly in front of) its parent star. Kepler has found a lot of planet candidates this way — well over 1200!
Kepler-16 is one (OK, two) of those stars (hence the name), located about 200 light years from Earth. The two stars are eclipsing binaries, meaning that we are viewing them from Earth in the plane of their orbit. Twice every orbital period, one of the stars blocks the light from the other and we see the total light from the system dip a little bit. We know of a lot of eclipsing binaries, and their properties are pretty well understood.
But Kepler-16 is different. Read More
Two stories just came out that I would love to spend time writing up in full, but I’m trying to get a million things done before I leave for Dragon*Con in the morning, so I’ll be brief:
1) Astronomers using the Chandra X-ray Observatory have discovered a binary black hole: two ginormous beasts orbiting each other about 500 light years apart in the center of the gorgeous spiral galaxy NGC 3393. Each has a mass of at least one million times that of the Sun. While binary black holes in the centers of galaxies have been spotted before, this is the closest one found: a "mere" 160 million light years away!
2) A newly-discovered planet (PDF) orbiting a star just 36 light years away appears to be at just the right distance to potentially have liquid water on its surface. The planet, HD85512b, orbits a star somewhat smaller and cooler than the Sun, but close enough to it that it actually gets more heat on average than Earth does. The planet is hefty, 3.6 times the mass of the Earth, but the size is not known (you get that from transit data, which we don’t have, and it would give us an idea of the surface gravity). So we don’t know anything about it, really, but if conditions are just so, it has the best potential we know yet for a planet with liquid water. Nat Geo has a great writeup of all this.
Now, if the Universe would kindly oblige not doing anything interesting for a few more hours, I can finish packing!
There’s some chatter on the web right now over a new scientific paper about a nearby exoplanet, and what I’m seeing are people speculating that it might be earth-like. Technology Review even titled their article "Astronomers Discover Habitable ExoEarth Orbiting Binary Star".
The problem with that is that the planet’s not terribly earth-like, and it may not be habitable*.
So what’s the deal? I read the journal article (PDF), and this really is a good story, just not the one I’m seeing the chatter about.
55 Cancri is a nearby binary star at a distance of about 40 light years. One star is a dinky red dwarf, and the other is a fairly Sun-like star, though somewhat smaller and cooler. It’s also much older, roughly 10 billion years old, more than twice the age of the Sun. It’s actually at the point where it’s starting to evolve into a red giant, and is called a sub-giant.
Back in 2007 it was announced that at least five planets orbit the bigger of the two stars (called 55 Cancri A; confusingly the red dwarf is 55 Cancri B (note the capital letter), while the planets are called b-f (lower case)). They range in mass from 0.026 to 3.84 times that of Jupiter (8.3 to 1200 times the mass of the Earth). 55 Cancri e is the lowest mass of these, but is extremely dense and hot, so not at all earth-like.
55 Cancri f is the interesting planet, though. The astronomers in question observed the star using an interferometer, allowing extremely precise measurements of the star’s size, which in turn yielded very accurate numbers for its temperature and mass. All these together can be used to figure out its "habitable zone", the region around it where an orbiting planet would have liquid water on its surface.
Now right away, I’ll say that finding the HZ (as we in the know call it) is not really straightforward. For example, a planet that has a thick atmosphere can be farther from its star and still have water due to the greenhouse effect; in fact, without air the average surface temperature of the Earth would be below freezing! And the greenhouse effect depends on what’s in the atmosphere, its density, and so on. So I am wary of any declarations of planets being habitable based on this alone.
Still, let’s see what we get. Read More