Hey guys, there's going to be a lull in IP posts for about a week (but hopefully less!), as I work to move the site over to Wordpress (from Typepad). I'll try to get this done as quickly as possible, and once it's done, I'll be back with your usual dose of One O'Clock awesomeness. I'll see you all on the other side!
February 14, 2011
February 11, 2011
One O'Clock Daily - The Inside Scoop on Kepler
Hey guys, I've got a treat for you all today. A good friend of mine happens to be one of the researchers pouring through the data from the Kepler mission that we've all heard so much from over the past week. She agreed to come on as a guest writer and do a piece on just what Kepler found out there, and what it means for our understanding of the galaxy. So, without further ado, here she is:
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Over the past week, you have probably heard a lot of exciting news about Kepler and extrasolar planets. Today, I would like to take a moment to summarize these findings and explain why these discoveries are so important, and so cool! But first, if you haven’t been following the news, or have no idea what Kepler is, let me give you some background info. The Kepler Mission is a NASA funded satellite whose goal is to search for extrasolar planets (exoplanets) by way of the transit method. In other words, the Kepler satellite is continuously staring at approximately 150,000 stars waiting to see if a planet crosses in front of its star, dimming the starlight. This dimming of the star’s flux is extremely small, for example, Earth only dims are Sun by 0.01% when we transit it, and Kepler is built for this precision. What we end up receiving from Kepler is a transit curve (check out the animation here for a great example). For more information about Kepler, I recommend visiting the Kepler website.
Artist rendition of Kepler 10b. (Credit: NASA)
So far, Kepler has discovered 18 exoplanets, but I am only going to comment on Kepler 10b and the Kepler 11 system. Kepler 10b was the first rocky planet found by Kepler and is the smallest planet found thus far. Its radius is only 1.4 Earth radii, and has a mass of 4.6 Earth masses. However, its average density is 8.8 g/cm^3 compared to Earth’s 5.5 g/cm^3. What this means is, this little planet may be composed of an iron core that is about 75% of its mass. The rest of this planet is just rock, no water, no hydrogen or helium atmosphere, just boring rock. Except, at only .017 AU (1 AU equals the distance from Earth to the Sun), it’s likely that the entire surface is completely molten. In fact, the equilibrium temperature is a warm 1800K, but since the planet is in a synchronous orbit (one side always faces the star) the day side is even hotter than this. The surface of this planet, at least on the day-side, is likely to be molten. It is also hot enough to evaporate the rocky surface creating a silicate (rock-composition) atmosphere! Not a habitable planet by any means, but definitely a planet that has a potential to contain a unique atmosphere.
Kepler 11 System, in comparison to our solar system. (Credit: NASA)
The most recent Kepler discovery is that of the Kepler 11 system, which contains 6 planets (Kepler 11b-Kepler 11g).
A quick note about planetary naming systems, the first planet discovered in a system is named with either a mission’s or the star’s name + the lowercase letter b at the end. For example, Kepler 10b, CoRoT 7b, or HD80606b. The next planet discovered in the system is denoted with the letter c, Kepler 11c, and so on. This does not necessarily mean the planets are named b, c, d in order from the distance of their star, but simply by which one was discovered first!
These planets orbit a sun-like star, and seem to have many similar properties to our solar system: a couple rocky planets followed by several gas giants in a very coplanar (non-inclined) orbit. The only difference is, five of these six planets have been shoved into orbits closer to their star than Mercury is to our sun! And the sixth planet is closer than Venus! No other system that we have discovered to date is even close to replicating the characteristics of this packed system. Furthermore, the planets themselves are unique. Because their orbits are so close, these planets gravitationally perturb each other’s orbits leading to a “transit timing variation,” or that the transits don’t occur periodically. From these perturbations, the Kepler team has accurately been able to calculate their masses, which helps narrow down their compositions. Kepler 11b and 11c lie at a distance of 0.09 and 0.1. Their orbits are extremely circular so they will never collide, but they are also not in orbital resonance, which is unusual.**orbital resonance** Most formation models predict that if planets migrate inward as they form, they must end up in an orbital resonance, yet these planets might prove these models wrong. These two planets are 1.97 and 3.15 Earth’s radii respectively, and have a density slightly less than that of rock. If these two planets have an atmosphere it is quite possibly composed of steam or a mixture of silicates if the surface is hot enough. The next three planets, Kepler 11d, 11e, and 11f, are grouped together at distances of 0.16, 0.19, and 0.25 AU. They have radii between 2.6 times Earth’s radii and 4.5, with masses ranging from 2.3 to 8.4 Earth masses. The craziest part is that all three planets posses densities that are less than 1 g/cm^3 (density of water = 1g/cm^3). One compositional possibility is that these planets have a small core surrounded by a large hydrogen or helium atmosphere such as Saturn. However, some scientists believe that these planets are not massive enough to have the gravitational pull necessary to contain a stable H/He atmosphere. The other possibility is that these planets have a large steam atmosphere with some H/He in the atmosphere. The last planet, Kepler 11g is further away, at 0.46 AU, and does not appear to have its orbit perturbed by the other planets. This being the case, there is no mass recorded for this planet, and therefore no density, but the planet does have a radius of 3.7 Earth radii.
So how did this crazy system form? Well...scientists have no idea! There is a large argument about whether these planets migrated or formed where they are. Both theories have pros and cons, but this system has definitely caused many theorists to rethink planetary formation. But one question to consider, why doesn’t our solar system resemble this system or did we at one time have 6 planets orbiting our Sun inside Mercury’s orbit?
Besides these 11 confirmed systems , the Kepler team also announced last week that they currently have 1,235 planetary candidates. In order to be a confirmed planet, astronomers use the radial velocity method as a follow-up to measure the planet’s mass and rule out any false positives. Regardless, if even 50% of these planetary candidates are confirmed, that is still more exoplanets than we have found to date. 68 are these candidates are Earth-sized and 288 are slightly larger than Earth. 54 candidates lie in the habitable zone of their star (where water can exist in liquid form) and 5 of these are Earth sized! The other 49 habitable zone candidates range between twice the size of Earth to larger than Jupiter (which brings up the possibility for habitable exomoons, like Pandora). One of the implications of this discovery is the number of planets out there. As a back of the envelope calculation: Kepler surveys 156,000 stars in the sky and there are (extremely roughly estimated) about 30 billion sun-like stars in our galaxy. Assuming that half of the planetary candidates are actually planets, and this is the norm, a quick calculation shows that there should be over 118,000,000 planets in our galaxy. Also, assuming those 5 Earth-sized candidates in the habitable zone is the norm, that means there should be over 960,000 Earth-sized planets in the habitable zone in our galaxy! Furthermore, the transit method does not find planets outside of a couple AU as it isn’t accurate enough, so the real number of planets is likely to be much higher! Of course this is all based on assumptions, but definitely makes you wonder, are we alone in the galaxy or is their intelligent life on one of those 960,000 habitable planets?
Kepler Exoplanet Candidates from blprnt on Vimeo.
For more information, and the references I used, please check out the following links:
- Kepler 10 paper: http://arxiv.org/PS_cache/arxiv/pdf/1102/1102.0605v1.pdf
- Kepler 11 paper: http://arxiv.org/ftp/arxiv/papers/1102/1102.0291.pdf
- All of NASA’s Kepler-related press releases can be found at: http://kepler.nasa.gov
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Jessica Roberts has a degree in planetary science from Caltech and works on the Kepler and MOST datasets at the University of Idaho. She enjoys basketball, running, and her dog, and thinks that extrasolar planets are just about the most awesome things ever. She was once described as a "Star-crossed nerd seeking transiting exoplanet", and her future plans involve finding ways to spread her love of science to the next generation of students, while simultaneously trying to answer every question there is.
February 09, 2011
February 08, 2011
One O'Clock Daily - A Pathway to Space, part 2
Biz Dev Guy: We could make a preposterous amount of money from communications satellites.
Engineer: It will be expensive to build those, but even so, nothing compared to the cost of building the machines needed to launch them into orbit.
Biz Dev Guy: Funny you should mention that. It so happens that our government has already put $4 trillion into building the rockets and supporting technology we need. There's only one catch.
Engineer: OK, I'll bite. What is the catch?
Biz Dev Guy: Your communications satellite has to be the size, shape, and weight of a hydrogen bomb.
From Space Stasis, by Neal Stephenson for Slate
That about sums up Neal Stephenson's point. Our current space launch systems are descended from ammunition, and somewhere in there, because of that, they tend to be not fully reusable and expensive. Very, very expensive.
But there are alternatives. Stephenson speaks of "no shortage of proposals for radically innovative space launch schemes that, if they worked, would get us across the valley to other hilltops considerably higher than the one we are standing on now—high enough to bring the cost and risk of space launch down to the point where fundamentally new things could begin happening in outer space. But we are not making any serious effort as a society to cross those valleys. It is not clear why.".
Let's take a look at some of those proposals. For now, I'm only going to pick three, in increasing order of required R&D.
The Skylon is probably humanity's best bet for a fully reusable spaceplane. With scarce funding—the whole project is estimated to cost roughly $12 billion—Reaction Engines Limited, the British company behind the Skylon design, has been focusing on developing the SABRE engines required by the spaceplane. Engineering estimates suggest that the Skylon could bring launch costs down from ~$12,000 / pound to as low as $450 / pound, a 25-times improvement over current technologies.
Magnetic levitation launch assist
Another idea involves providing the launch vehicle with a kinetic energy boost at take off. The theory goes that if you can accelerate your launch vehicle to some starting velocity with an external system, then it will have to carry much less fuel to reach the desired orbital velocity, making your total system that much more efficient on the margin. The most popular idea for a launch assist system involves magnetically levitated sleds that the launch vehicle rides on. These sleds would be accelerated along a track to a final velocity of between 1,000 km/hr (as in the example linked in the title) and 10,000 km/hr. At the end of the boost phase, the tracks would angle upwards and the launch vehicle would shoot off, igniting its own engines, and propelling itself the rest of the way into orbit. If you can build such a system at high altitudes, you would gain additional efficiency from the thinner air at the launch site, so most such proposals involve building sites near Kilimanjaro or in the Ecuadorian highlands. It's important to note that your launch vehicle would still need some power of its own. Earth's dense atmosphere means that you likely couldn't launch your vehicle at orbital velocity right off the tracks. These systems would require more R&D effort than a spaceplane like the Skylon, but are still based on technologies that are only evolutionary steps above and beyond what we have today. The magnetic-levitation techniques, the rocket engines, and even the heat shielding have all been demonstrated at near the required performance, the main difficulty would lie in integrating them all into one system.
Space elevators are exactly what they sound like: a giant elevator cable, with little elevator cars running up and down it, reaching all the way into orbit. The tricky part of keeping such a structure taught and extended against the Earth's gravitational field is usually solved by tethering the far end to a captured asteroid, so that the center of mass of the elevator lies in geosynchronous orbit, and the centrifugal force of the whole assembly keeps it from collapsing. Undoubtedly, a space elevator is the most efficient form of launch technology I know of. The elevator cars don't have to carry any of their own fuel, and they can travel slowly enough up the cable that they don't need to worry about atmospheric drag. The result is that all of the energy required for moving cargo into orbit can be generated via a coal, natural gas, nuclear, or renewable energy plant on the ground (the most efficient form of energy production), carried up the cable to the car in the form of electricity, and immediately used to put to work against the Earth's gravity by simple, and very efficient, electric motors. The trouble with space elevators, however, comes from the required strength of the materials that would make them. Because of the Earth's relatively strong gravity, we would require carbon nanotubes to build such a device for our planet. Unfortunately, the current record-longest nanotubes measure only centimeters long, a far cry from the thousands of miles that a space elevator must stretch. But once we've tackled that problem, I expect that space elevators will open up orbit like never before.
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