Tag-Archive for » Kepler «

NASA review of space astrophysics missions extends all 9!

Author: Apr 03

Well, this is some very welcome and happy news: NASA’s 2012 Senior Review for Operating Missions has recommended to NASA that eight of the nine operating space-based astrophysics missions be extended in funding through fiscal year 2016, and NASA has complied!

Holy wow.

This really is great news! The missions extended through FY 2016 are Hubble, Chandra, Fermi, Planck, Suzaku, Swift, XMM-Newton, and Kepler. The exception is the infrared observatory Spitzer, which ran out of coolant a few years ago but is running in an extended "warm" phase, still able to do science. It will be extended through 2015, which is earlier than hoped, but it could be worse. The details are in the report issued by the Senior Review (PDF).

I’m very excited specifically about Swift — a gamma-ray burst mission that I worked on years ago, and which has been operating for more than 7 years so far. But I’m even more excited about Kepler. This is fantastic ...


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A planet boils away under its blow-torch star

Author: Jan 19

There’s been a lot of exoplanet news lately! Part of that is due to the American Astronomical Society meeting recently — in fact, there was so much I wrote four articles just from that (Part 1, Part 2, and Part 3, and Part 4). This next story wasn’t released at the meeting, yet may honestly be the most mind-blowing of them all.

Astronomers have found what appears to be a planet literally boiling away from the blast-furnace heat of its star.

Holy cosmic oxyacetylene torch!

[Image: Reign of Fire by the extremely talented space artist Inga Nielsen. She has prints of them for sale, too!]

There’s a bit of a back story here. The star, KIC 12557548, is about 1500 light years away, and is one of many thousands being observed by the orbiting Kepler Observatory (KIC stands for Kepler Input Catalog, a list of stars under Kepler’s watchful eye). The observatory stares at one spot in the sky, looking for stars whose brightness dips periodically. There can be many causes of such behavior, one of which is the presence of planets orbiting the star and blocking the light from it as they pass in front of it. This is called a transit, and has proven to be wildly successful; hundreds of planets have been discovered this way.

What the authors of this new study are saying is that they see a periodic dip in the brightness of KIC 12557548 every 15.685 hours. Yes, hours. The star is a bit smaller and cooler than the Sun (a K star with about 0.7 times the mass of the Sun, if you want specifics), but even so, the planet must orbit the star a mere 1.5 million kilometers (900,000 miles) from its surface — that’s less than four times the distance of the Moon from the Earth!

That’s close. You’d expect the planet to be cooking… and you’d be right. It’s probably somewhere around 2000°C (3600°F).

Usually, with most planets, the amount of light blocked as the planet passes in front of the star is the same every time. That makes sense, because the planet itself isn’t changing. But not for KIC 12557548. What they saw was that every transit was different. Sometimes more than 1% of the light is blocked, sometimes they detect no dimming at all at the appointed time. That’s really weird.

They looked at and eliminated a few different scenarios, but the fact that the planet is that close to the star really leaves just one idea: a rocky world, probably half the diameter of Earth, being vaporized by the heat of its parent star*.

Yegads.


It sounds crazy, but it fits a lot of their data. The planet has to be small, or else its gravity would be enough to hold onto the hot material. The outflowing material is thick and dusty, so it can block the light from the star. Changes in the amount of light blocked are due to changes in the dust cloud, which themselves may be due to inhomogeneities in the planet itself; different layers and features being vaporized. Sometimes there’s no dust at all, and the planet is too small to be detected blocking the star, so no dip in light is seen.

Given all this, it’s possible to estimate how much mass is lost by the planet, and the answer is a staggering 100,000 tons per second. That sounds like a lot — and it is! — but planets are pretty big, even "super-Mercuries" like this one. It would take hundreds of millions of years to totally evaporate the planet at that rate, so it’ll most likely be around for a while for us to study.

How amazing would it be to be there (protected thoroughly, of course)? Imagine: from the planet, the star eats up half the sky, hundreds of times bigger than the Sun looks from Earth. Blazing down, fierce, uncompromising, the heat is intense and all-encompassing. The surface of the planet is glowing, liquid lava. There’s an atmosphere, but it’s composed of vaporized rock and it’s as hot as the devil’s breath. Flowing upward under the weak gravity, the dusty gas itself glows from heat as it rises up into space. The wind and pressure from the light of the star itself pushes on the gas, forming a long, graceful curve like the tail of a comet, stretching for millions of kilometers behind and away from the planet.

It’s a disaster on a planetary scale, an entire world boiling away in front of your eyes! And it will only end when the planet itself is gone, evaporated, vaporized.

Pfffffft!

The more I read about exoplanets, the more I wonder what surprises are in store for us. We’ve only just started looking for them! We need bigger telescope, better equipment, and simply more eyes on the sky. What other crazy amazing thing is out there just waiting for us to see it?

Tip o’ the Galactus helmet to Dan Vergano at USA Today. Image credits: Inga Nielsen, Gate to Nowhere; NASA, European Space Agency, Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS).

* I’ll note that another planet, HD 209458b, is so hot that its atmosphere is puffed up and is being lost to space, like a gigantic comet. But that’s the atmosphere; for this planet we’re talking the actual surface being boiled away. Craziness.


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Exoplanet news part 1: I shall call it Mini Solar System

Author: Jan 12

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

Related posts:

- 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


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No, SETI has not detected an alien signal from a Kepler planet

Author: Jan 06

Last night, I started getting emails and tweets asking about a possible detection of a radio signal coming from two of the newly-discovered planets orbiting other stars.

Cutting to the chase: yes, a signal has been seen, but no, it’s not coming from some alien civilization. It’s almost certainly something much closer, like a satellite interfering with the observation.

So what’s the deal?

You talkin’ to me?

The Search For Extraterrestrial Intelligence (SETI) is a privately-funded group of scientists and engineers who are trying* an ongoing effort to figure out ways to detect signals from space that could be coming from other intelligences: aliens. They focus (haha) mostly on radio signals, since it’s very easy to send radio waves across the vast light years separating stars, it’s easy to detect radio waves (so primitive life like us can pick up the call), and it’s easy to encode information that way. Heck, we’ve been broadcasting coded radio waves for over a century now!

Currently, no unambiguous alien "Hello there!" has been detected. The sky is big, there are a lot of stars out there, and the radio spectrum is really wide, too. Think of how many radio stations there are on a typical radio dial from top to bottom; now divide that up into a billion tiny slices and try to find the one that’s playing the song you want to hear. It’s something of a painstaking process.

Recently, astronomers came up with a clever idea: the Kepler space mission is finding tons of planets orbiting other stars. It may find an Earth-like planet orbiting a Sun-like star at just the right distance to allow life to evolve, though no such planet has been found just yet. Still, why look all over the sky when we know where there are lots of planets?

Can’t stop the signal

So a search targeting those stars with planets has been set up. And that’s where our story picks up: using the ginormous 100 meter Green Bank Telescope, astronomers from UC Berkeley found what look like artificial signals when observing two different stars. The stars are called Kepler Object of Interest 812 and 817 (or just KOI 812 and 817 for short). Here’s an example of a signal they found from KOI 817:

This takes a little explaining. Basically, the plot shows how strong the signal was (shown by how bright the line is) over time (which starts at the top of the graph and increases downward) versus the frequency of the signal. So the observation starts at the top, and you can see the strength go up and down as time goes on. If the signal had been at one strength the whole time, the line would be equally bright everywhere. It also is tilted, which means it changed in frequency. A radio station sitting on Earth broadcasts at one frequency that doesn’t change (which would be a vertical line on this plot), but if the radio source is moving toward or away from you, the frequency will shift via the Doppler shift (the same thing that makes a train whistle change pitch as it heads toward you, passes, and then heads away).

Right away there’s a lot you can figure out at a glance! A shifting signal means it’s probably in space and not on Earth. More importantly, it’s very narrow in frequency, whereas a lot of natural signals emit radio all over the spectrum, so instead of a narrow line you’d see a very broad line. So it’s likely to be an artificial signal from space.

Hey, WOW. Could it be aliens?!

… and they is us

Well hold on there, buckaroo. There is another source of artificial signals from space: us. We have lots of satellites orbiting the Earth, and they broadcast quite loudly in radio waves. They use narrow frequency bands, and would exhibit a Doppler shift as they orbit.

So how can we tell the difference between aliens and humans in space? One way is to move the telescope! If you’re pointed at an alien transmitter, then moving the telescope will point you in a different direction, and the signal should go away. On the other hand, strong satellite signals can be detected by radio telescopes even when they point in another direction; the signal can leak into the telescopes even when you’re pointed well away. This is called "side lobe interference", and it’s a major pain for radio astronomers when they’re observing faint objects.

But in this case it’s actually helpful: the astronomers moved the telescope, and yet they continued to see these signals. So it’s clear these signals are coming from our own satellites orbiting the Earth, and not Klaatu, Klingons, or Klendathu.

So, sadly, it’s not aliens. But the good news is the method works! The astronomers use automated software to look for what could be artificial signals, and the computers flagged these detections. As the project ramps up and they start looking in earnest at the hundreds or even thousands of exoplanets orbiting stars out in the galaxy, they’ll certainly find lots of signals like these — interference from somewhat closer and more mundane sources.

But, as the search goes on, who knows? If Vulcans or Gallifreyans (but hopefully not Cardassians or Daleks) are out there, and they’re trying to catch our attention… well, we’re listening.

Tip o’ the spacesuit helmet to vjekoslr on Twitter. My thanks to Seth Shostak and Andrew Siemion for helpful comments. Signal plot courtesy The Search for Extra Terrestrial Intelligence at UC Berkeley, GBT image courtesy NRAO/AUI.

* I tried to distinguish between SETI (the search) and the SETI Institute, but decided this could be made clearer. Sorry about any confusion.

Related posts:

- Giant spaceships to attack December 2012?
- No, a new study does not show cosmic-rays are connected to global warming
- No, the “supermoon” didn’t cause the Japanese earthquake
- No, there’s no proof of a giant planet in the outer solar system
- No, a pole shift won’t cause global superstorms


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Video chat about the new Kepler planets

Author: Dec 21

Yesterday, astronomers announced the discovery of a compact solar system orbiting a distant star, with two of the planets being very close to the size of the Earth.

My friend Fraser Cain, from Universe Today, put together a video chat Q&A about the discovery with me, Nancy Atkinson from UT, Emily Lakdawalla from the Planetary Society Blog, and Alan Boyle from the MSNBC Cosmic Log blog. We talked about the discovery, how it was made, what it means for exoplanetary science, and a few other topics just for good measure.

Fraser did this using Google+ Hangout, the social network’s video chat software. They rolled out a new feature just a few days ago where a few people can chat on camera, and the whole thing can be broadcast on G+ at the same time. Not only that, but, obviously, it can be recorded and uploaded to YouTube as well. This is brand new stuff, and not widely available just yet, so we had some issues with it (notably Fraser’s window never was displayed on the main screen; the images he displays at 13 minutes in can be found on the Kepler website).

Since I’ve got you here, there’s one very cool thing I’d like to expand on. Later in the video, we chatted about the physical characteristics of the Kepler-20 system, including how the planets’ orbits were tilted, and how you can determine that from the Kepler data. I poked around on the web afterwards, and found that the Kepler site has an amazing feature; an interactive display of all the confirmed planets they’ve found. For example, here’s the one for Kepler-20-f, the outermost of the five planets in the system, and the one closest to the size of Earth.

You can watch an animation of it going around the star, with a display of how it blocks the light. You can also see how the orbit is slightly tilted to the line of sight, and how it cuts a chord across the star. It’s truly a splendid way to show folks what they’ve found, and I highly suggest playing around with it (though it may be slow due to heavy use right now). When it loads, click the button labeled "Perspective" and then click "go to view from Earth". That’ll show you how a transit works pretty well.

From those pages, I found that these planets do orbit their star almost — but not quite — edge-on. An orbital inclination of 90° would be edge-on, and the planets, in order from the star, have tilts of 86.5, 88.4, 89.6, 87.5, and 88.7°. I was surprised to see that there is a spread of even as much as 3°. I wonder why? The planets probably formed farther out and migrated in toward the star; we know planetary migration happens for many (if not all) solar systems when they’re young, including our own. As these planets got closer, they could interact more strongly via gravity. Maybe that amplified their tilts somewhat. Or maybe I’m totally wrong in thinking the tilts should all be aligned in the first place.

We’re still new at this game, so there’s a lot left to learn. But that, my friends, is where the fun is. May we have lots more fun systems like Kepler-20 to investigate.


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Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!

Author: Dec 20

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! The others are much larger than the Earth: named Kepler-20b, c, and d, they have diameters of 24,000, 40,000, and 35,000 km (15,000, 24,600, and 22,000 miles); smaller than Uranus and Neptune, but still pretty hefty. We do have the masses for them: 8.7, 16.1, and about 20 times the mass of the Earth. Call them "super-Earths" if you like.

All these planets huddle pretty closely to their star; the orbit of Kepler-20f, the farthest from the star, would still fit comfortably inside the orbit of Mercury! Oddly, the configuration is very different than our own solar system. While ours has the lower-mass planets close in and the bigger ones farther out, in the Kepler-20 system they alternate, going big-little-big-little-big.

So how do we know all this? The Kepler observatory is in space, staring at one patch of sky all the time. There are 100,000 stars in its field of view, including Kepler-20. If there are planets orbiting a star, and we see the orbit of that planet edge-on, then once per orbit the planet directly passes between us and the star, blocking its light a little bit. This is called a transit, and the bigger the planet, the more light it blocks. That’s how the sizes of the five planets were found.

As these planets orbit their star their gravity tugs on it, and that can be measured by carefully observing the star’s light. As a planet pulls it one way and then another, there is a very small Doppler shift in the starlight, and the amount of that shift tells us how hard the planet is tugging on the star, and that in turn depends on the mass of the planet. Only the three bigger planets in the Kepler-20 system pull hard enough for us to measure, which is why we don’t have the masses of 20e and 20f; they’re too small to measure.

Also, to be clear, we don’t have direct images of these planets (those pictures above are drawings). They were found indirectly by how they affected their star. But these methods are now tried-and-true, and the existence of these five planets has been confirmed. They’re real.

This is a fantastic discovery for so many reasons: the smallest planets found orbiting another star, the first Earth-sized planets seen by Kepler, both in the same solar system, and in such an oddly-configured and compact system at that. This means we need to think more about how such planets can form, of course, since it’s so weird… but no matter what, it means we’re that much closer to finding the ultimate goal: an Earth-sized planet orbiting a Sun-like star in that star’s habitable zone, where liquid water can exist.

Every time I hear news like this, I wonder how much longer we’ll be waiting to hear that news… and I strongly suspect it won’t be too much longer.

Image credit: NASA/Ames/JPL-Caltech

Related posts:

- Kepler confirms first planet found in the habitable zone of a Sun-like star!
- A boiling superEarth joins the exoplanet roster
- Big news: first “solid” exoplanet found!
- Two exoplanets discovered by "citizen scientists"


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Kepler confirms first planet found in the habitable zone of a Sun-like star!

Author: Dec 05

[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!]

http://www.eso.org/public/images/eso1134b/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.


Kepler detects planets when they transit their star, passing directly in front of the star, blocking its light a little bit. The bigger the planet, the more light it blocks. The astronomers going over the data determined that Kepler-22b is about 2.4 times the diameter of the Earth. The problem is, that and its distance from its star are all we know. We don’t know if it’s a rocky world, a gaseous one, or what. It may not even have an atmosphere!

So we can’t say what conditions are really like on the planet. As you can see on the diagram above (click to embiggen), mass and atmosphere make a difference! Venus and Mars are both technically inside the Sun’s habitable region, but Venus’s thick atmosphere makes it hotter than an oven, and Mars’s thin air makes it colder than a freezer (if we could swap those two, though, then things would get pretty interesting in the solar system). Kepler-22b might be a paradise, or it might be the opposite. That depends in part on the planet’s gravity, and for that you need its mass.

The problem is we don’t know the planet’s mass. Kepler’s transit technique doesn’t find that; it has to be determined using very tricky observations of the planet’s gravity as it tugs on its star. Since Kepler-22b’s orbit is 290 days long, that makes this kind of observation much harder (the closer in the planet, the harder it pulls on the star, and the bigger the signal… plus you don’t have to wait as long for it to go around once). In fact, that’s why it took so long to confirm this planet’s existence — you need three transits for that! The first transit took place coincidentally just a few days after Kepler was launched in space, but the planet had to pass in front of its star a second time 290 days later to make sure the dip in starlight was real, and then a third time 290 days after that to confirm the period.

Now we might speculate that the planet is similar in composition to Earth — rock and metal and water. If so, then it has more gravity than Earth. Standing on its surface (assuming it has one) you’d weigh 2.4 times what you do on Earth! So even if it looks like Earth, it won’t be too much like it. On the other hand, if it’s made of lighter materials, the gravity could be lower. We just don’t know yet.

But that doesn’t diminish the fact that this planet exists at all! We’ve found a relatively low mass planet orbiting its star at the right distance for Earth-like conditions to arise. That’s very exciting! We’ve been closing in on this for a while now, finding lower mass planets that are too close in (though getting close to their star’s habitable zone), or bigger planets (like Jupiter) at about the right distance. This is the first confirmed planet that’s at the right distance and may have the right mass to be considered Earth-like.

Right now, as far as we know, Earth is still the only planet in the whole galaxy that has life, or even the exact right conditions for life. But what observations like this tell us is that there are more planets out there, a huge variety of them. This means the odds get better every day that — for some planets at least — the characteristics all come together to make them look an awful lot like us. The day is getting closer when we find that first planet. It’s just a matter of time, and the diligent work of astronomers like those who found Kepler-22b.

Image credits: ESO/M. Kornmesser; NASA/Ames/JPL-Caltech

Related posts:

- How many habitable planets are there in the galaxy?
- A boiling superEarth joins the exoplanet roster
- New study: 1/3 of Sun-like stars might have terrestrial planets in their habitable zones
- 50 new worlds join the exoplanet list


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A boiling superEarth joins the exoplanet roster

Author: Nov 30

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. This implies it’s denser than our home world. Why? Because at 1.6 times our diameter it has 4 times our volume (volume increases with the cube of the diameter, and 1.6 x 1.6 x 1.6 is about 4). If it had the same density as Earth that would mean it would have 4 times our mass. Since the mass is likely higher than that, it is probably denser.

That makes sense to me. The planet is orbiting its star at a distance of only about 6 million km — far closer than Mercury orbits the Sun! That makes the surface of Kepler21b hot, probably about 1900 K (roughly 1600°C or 3000° F). That kind of heat tends to boil away lighter stuff like water, leaving denser material behind (unless the planet is big enough to hold onto those lighter materials; Jupiter could, for example, but this planet is far smaller). So the planet being denser than Earth isn’t surprising.

Playing with the numbers a bit, I find the surface gravity of the planet is 4 times Earth’s, too. If you weigh 120 pounds on Earth, you’d weigh nearly a quarter ton on Kepler-21b. Not a great place to lose weight!

… on the other hand, with a surface temperature literally high enough to boil lead, you’d lose weight fast. But then, you’d be a puff of vapor. Probably not the best diet plan.

Not that you’d be heading over there anytime soon anyway. Kepler-21b is 350 light years away, or 3.5 quadrillion kilometers (2000 trillion miles) away. Getting there would be tough. I suggest something easier, like doing 10,000 push ups a day.

Anyway, this is an amazing detection; the planet is pretty small, very far away, and its parent star very luminous. These all combine to make this a tough world to detect, but that goes to show you: we’re getting really good at this sort of thing.

How long before we find another Earth this way? I’m guessing not very long. A few years at most. If they’re out there, they can’t hide forever.

Credits: ESO/L. Calçada; Steve Howell and the Kepler team

Related posts:

- Motherlode of potential planets found: more than 1200 alien worlds!
- Kepler works!
- Astronomers discover a wretched hive of scum and villainy
- Two exoplanets discovered by “citizen scientists”


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New study: 1/3 of Sun-like stars might have terrestrial planets in their habitable zones

Author: Sep 29

A paper has been accepted for publication in a science journal (PDF) where the author has analyzed data from NASA’s Kepler planet-finding observatory, trying to figure out how many Earth-sized planets there might be in the galaxy orbiting their stars in their habitable zones; that is, at the right distance so that the star warms the planet enough to have liquid water. In the paper, he estimates that on average 34% (+/-14%) of Sun-like stars have terrestrial planets in that Goldilocks zone.

34%!

I can explain how he got this number. But I can also explain why I think this needs to be taken with a grain of salt. Let me be clear: it’s possible he’s right, and I suspect he may very well be. His math looks good to me. But a couple of assumptions he had to make need to be pointed out, and I want that to be clear before the media start running around saying there are billions of Earths in the galaxy based on this.

Here’s the deal. Kepler is an orbiting observatory that’s staring at about 100,000 stars, looking for dips in their light when an orbiting planet passes in front of them from our perspective. The length of time the dip takes gives us the orbital period of the planet, and the size of the planet (if the star’s size is known, generally true) can be determined by how much light is blocked. I talk about how this works in a little more detail in an earlier post.

The astronomer, Wesley Traub of Caltech, based his analysis on only the first few months (136 days) of Kepler data, what was available at the time. This introduces a bias into the calculations, because that length of time is too short to conclusively find planets in their stars’ habitable zones! Even being generous, the length of such an orbit is at least 200 days, much longer than the Kepler sample. So he was forced to look at only short-period planets (with periods of 42 days or less), much closer to their stars, and extrapolate the data from there. I’ll note that Dr. Traub was up front about potential biases in the data and his analysis.

He looked at stars similar to the Sun (with a range from somewhat hotter to somewhat cooler, roughly F, G, and K stars). He then looked at data for all planets detected — terrestrials (Earth-sized), ice giants (like Uranus and Neptune), and gas giants (like Jupiter), getting their size and orbital period.

http://www.eso.org/public/images/eso1134b/Then he found the ratio of terrestrial planets to all the planets seen. Again, remember, this ratio was found for planets somewhat close in to their stars.

Then he plotted all the planets versus distance from their parent stars. For example, you see very few planets very close to the star (probably because it’s hard to form or get a planet to orbit that close in), then more as you get farther out, then fewer at some large distance (which may, once again, simply be due to the fact that planets with long orbital periods can’t be seen in the short duration of the data). He then found an equation (called a mathematical fit) that did a good job predicting the shape of the plot. Once he had that, it’s easy enough to extrapolate it out to the distance of the habitable zones of the stars.

That gave him an estimate of all planets orbiting there, including gas and ice giants. Multiply by the ratio of terrestrial planets, and boom! 34% of stars like the Sun should have planets that are Earth-sized orbiting them at the right distance.

The thing that makes me most uncomfortable is that he had to use those short period planets, and extrapolate outwards. Extrapolation is always dangerous because you can’t be sure your fit behaves well outside the range in which you calculated it. For example, imagine you took a census of 1000 people ages 0 – 17, and made a fit to their height versus age. You’d find their height gets bigger with time, in general. But if you extrapolate that out to someone who is 40 years old, you might estimate they’ll be 4 meters tall!

We don’t know very well how planets form in their solar systems, and how they move around after. It may be that nature doesn’t make many planets in the habitable zone. Or maybe it does, but after some amount of time the planets move out of it, maybe through gravitational interactions with other planets. I’ll note that our own solar system makes that seem unlikely; we do have three planets in the Sun’s HZ!

So what are we to make of all this? I think Dr. Traub did careful, interesting work, and his number of 34% is probably not terribly far off. Again, we should keep our eyes on that number, since it’s based on extrapolation, but the calculation that went into it is well-reasoned. I wouldn’t be surprised if he’s pretty close to the mark.

And what does this mean for you, the science enthusiast? Well, F, G, and K stars comprise very roughly a quarter of the stars in the Milky Way, or something like 50 billion stars total (again, I’m being really rough here). That means, assuming Traub is correct, there could be 15 billion warm terrestrial planets in our galaxy alone!

Interestingly, last year I wrote about how another estimate indicated that 1/4 of Sun-like stars might have Earth-like planets. That’s not far from Traub’s ratio, and that 2010 study used different techniques on different stars! Not conclusive, but interesting.

I’ll add that I think this work was worth doing even this early on in the Kepler mission. This is a great first step in analyzing the massive amount of Kepler data, and putting a number on it that we really want to know. As time goes on, and Kepler sends back more observations, Traub’s work will have paved the way to work on planets with bigger orbits. I’ll be very, very curious to see which way that number moves as more data come in.

Image credit: Dan Durda; ESO/M. Kornmesser; NASA. Tip o’ the dew shield to Technology Review.

Related posts:

- Two exoplanets discovered by “citizen scientists”
- Motherlode of potential planets found: more than 1200 alien worlds!
- Kepler finds a mini solar system!
- How many habitable planets are there in the galaxy?


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Two exoplanets discovered by “citizen scientists”

Author: Sep 22

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:

That’s the data for KIC 10905746, the 2.65 Earth-mass planet with the 10 day orbital period. The top half shows the brightness as measured by Kepler; the star is a variable star which means it changes its own intrinsic brightness with a period of every few days. That makes this a difficult target! The red lines mark the transits spotted by the planet hunters; you can see where the brightness dips more than usual. Hard to see, aren’t they? But once you get the period of the planet, you can then "fold" the data, cutting it up into time intervals based on that period. Observations that were taken at different times — but show the planet in the same position relative to the star (for example, every time the planet is right in the middle of the transit) — can then be added together, cleaning up the noise. That’s in the bottom half, which shows all the transits stacked up (and with the star’s own changes mathematically removed). The dimming of the star’s light is much more clear — the red line is the best fit to the event. But note the scale: the planet only blocks 0.2% of the star’s light! It’s pretty amazing that we can see it at all.

Remarkably, both stars in this case were flagged as potential planet-bearers by the software, but also removed from the list to make followup observations by the same code! In the case of the star shown, it’s because the star was thought to be a giant near the end of its life, making followup observations and analysis too difficult. However, the human brain is pretty good at his sort of thing, and several people on the Planet Hunters site picked out the planetary transit.

I’ll note that the planet orbiting the binary star announced last week was also spotted by a Planet Hunter! The Kepler team had already spotted it as well, so they get the credit, but still. It doesn’t take a professional to find even the really weird planets out there.

Pretty cool, and good indicator that this citizen scientist project has a bright (or very slightly dimmed, I suppose) future ahead of it. And you can still participate! All you have to do is go to the Planet Hunters website, sign up, and get cracking. Who knows? Maybe you’ll find another world…

… and I have to wonder. An Earth-like planet orbiting a Sun-like star will have a very weak transit. A computer would have a very hard time picking that out of the data, but we humans are pattern-finding machines. Will the first exoEarth be found by a professional astronomer, or instead by some science enthusiast who decided one day to check out this Planet Hunters thing…?

Image credit: transit art: ESO/L. Calçada

Related posts:

- YOU can find extrasolar planets
- Astronomers discover a wretched hive of scum and villainy
- 50 new worlds join the exoplanet list
- Kepler finds a mini-solar system


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