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?
For more information, and the references I used, please check out the following links:
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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.
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