- The team of the Kepler mission has already discovered fifteen new planets, some of them the size of Earth, but they promise to give us much more.
- Eric Ford, Professor at the University of Florida and member of the project, talks about his work and about what is to come.
If there is any field of science that is thriving right now, that is the search for exoplanets, that is, planets orbiting stars other than the Sun. During the last years we have seen discoveries made at a frenetic pace; since the first exoplanet was discovered back in 1993, more than 520 of these planets have been found. The race is still on and now it’s aiming to the discovery of a planet similar to Earth, not only in size and composition, but also lying at the right distance from its star to allow liquid water on its surface. This distance is what astronomers call “habitable zone” or “goldilocks zone”, as it may make the existence of life as we know it possible.
At this moment there are many projects searching for exoplanets, both from Earth-bound facilities (e.g SuperWASP), and from space (e.g., CoRoT) but it’s NASA’s Kepler mission which seems to have better chances of finding “the new Earth”. And that is because at NASA they didn’t settle for just a mission to raise the count of exoplanets out there, but dared to ask the question “How common are other Earths in our galaxy?”
For this purpose, they have created the space observatory named Kepler (after the famous astronomer) and launched it into an orbit around the Sun using a Delta II rocket from Cape Canaveral, Florida, in March 2009. The “spaceship” has a planned lifespan of three and a half years. In simple terms Kepler is a one meter diameter telescope connected to a huge 95 megapixel digital camera, which watches from space a tiny fraction of the Milky Way. In this patch of sky, Kepler stares without blinking at more than 156.000 stars waiting for the signal that reveals the existence of a planet.
Video: Kepler Mission Launch. Source: NASA TV.
This signal is a minute reduction in the light from a star that occurs when a planet in its orbit passes in front of it, much like a tiny eclipse. This is what astronomers call a “transit”, and thus, this method of finding exoplanets is called the “transit method”.
Image: Artists’ rendering of Transit Method. Source: SETI Institute.
The advantage of using this method is that it is capable of detecting planet as small as Earth, as opposed to other methods that have been used to discover bigger and more massive planets. The downside is that in order for the planets to be detected, they must be positioned in a way that they cross in the line of sight of the observer. If a planet’s orbital plane has a different orientation, then we cannot see it. For this reason it is important to look at a great number of stars: to raise the odds.
Kepler is very good at detecting these dips in brightness. We are talking about drops in brightness of one part in ten thousand (1/10,000), and these objects are extremely far away. Scientists like this to the reduction in brightness of a car’s headlight caused by a flea crossing in front of it, observed from a distance of 100 yards.
Once these brightness dips have been detected, the astronomers have to wait to observe three transits, in order to be sure that they are observing at least two complete orbits of the planet around its star. In this way Kepler can measure the orbital period; i.e., the time that it takes for the planet to make a complete orbit around its star. As we know, our Earth needs one year to do so; therefore, at least three years of observation are needed to confirm the existence of a planet like ours, and so astronomers can begin to study its characteristics.
Studying the magnitude of the reduction in a star’s brightness and the orbital period, the radius of the planet and the distance from its star can be inferred. When we are talking about planetary systems with multiple transiting planets, more parameters can be gleaned, such as the stability and the dynamics of the system, the masses of the planets and the shapes of their orbits.
Previously, most of the planets orbiting stars other than the Sun found by the “planet hunters” are gas giants, the size of Jupiter or bigger. However, Kepler is designed to search for planets from 30 to 600 hundred times less massive, around the size of Earth (Jupiter is 318 times more massive than Earth).
In several years , once the Kepler mission is over and its data analyzed, which may take several years, the scientists hope to develop a new model that helps us to understand the presence of planets beyond the Solar System, determining their abundance, their range of sizes and orbital shapes, how frequently the planets are in the habitable zone, how many of them are in multistellar systems (with more than one star) and find out the typical properties of the stars hosting planetary systems. At the end of the day, Kepler will allow scientists to reset the exoplanetary model and place our Solar System into its context in the Galaxy.
Discoveries
The Kepler mission is unveiling the existence of a lot of worlds beyond the Solar System and forging new ideas about the diversity of planets out there. So far, fifteen new planets have been confirmed, six of them in the same system. Let’s take a look at some of the discoveries made by Kepler.
Five new giant planets: In April 2010, using data collected during the first three months of operation of the probe, it was announced the discovery of five new exoplanets, called Kepler- 4b, 5b, 6b, 7b and 8b. Not terribly cool names, admittedly. They are five big worlds with very short orbital periods, between 3.3 and 4.9 days. This means that these worlds make a complete trip around its star in only 5 days! This kind of planet is called “Hot-Jupiters” due to its big size and high temperatures, more than 2200ºF (1200ºC): hotter than molten lava, which implies that Earth-like life cannot exist on them. Those planets are orbiting stars bigger and hotter than our Sun. The research team expected to discover this kind of planets first, with very short orbital periods, which allowed Kepler to see a lot of transits quickly. These discoveries open the door for the arrival of smaller planets with longer orbital periods.
Image: Comparative Graphics of the sizes of the new planets and Jupiter and Earth. Source: NASA.A system with two planets: In late August 2010, the Kepler mission confirmed its first system with more than one planet: Kepler-9. This was not the first double system ever discovered, but it was the first time that the transit of both planets could be detected, called Kepler-9b and Kepler-9c. Also, for the very first time astronomers could detect small variations in the time of transits, caused by the gravitational interactions between the planets.
In order to obtain a higher precision in the characteristics of these planets, the astronomers made a series of Earth-based observations using the W.M. Keck Observatory in Hawaii. These observations demonstrated that Kepler-9b, the closest to the star, is larger than Kepler-9c and that both planets have similar masses, slightly smaller than Saturn’s. Kepler-9b has an orbital period of only 19 days and Kepler-9c completes an orbit every 38 days.
Image: Illustration of the system Kepler-9. The system is oriented edge-on, as seen by Kepler, with one of the planets commencing a transit in front of its star. Source: NASA/Ames/JPL-Cattech.
In addition to the two giant planets, scientists have found what seems to be a third planet from a less intense transit signal. This signal points out the existence of an object 1.5 times the size of Earth in an orbit very close to the star, with an orbital period of only 1.6 days. Only after a detailed analysis of additional data did the team become confident enough to announce it as a planet, Kepler-9d.
The smallest exoplanet: On 10th January 2011, the Kepler team announced the discovery of its first rocky planet. Previously, small planets had been discovered, bur their densities were not known precisely. By measuring small vibrations of the host star, Kepler was able to measure the sizes of the star and planet so precisely, that astronomers determined the planet to be rocky. This is the smallest planet around a sun-like star discovered to date and demonstrates Kepler’s capacities to detect planets the size of Earth.
Kepler-10b is a small rocky world that completes an orbit around its star in less than a day (0.84 days) and that is extremely close to it, 20 times closer than Mercury is to the Sun, and that’s why its temperature is very high, around 1800ºC – not in the habitable zone. It has been determined that the planet has a mass 4.6 times higher than Earth and a density of 8.8 grams per cubic centimetre, similar to an iron dumbbell.
In order to find the properties of Kepler-10b, additional observations of its parent star were made from the Keck Observatory in Hawaii. The star Kepler-10 is one of the brightest in all of Kepler’s field of view, so the scientists were able to characterize its properties with great accuracy, which enabled them to measure the properties of Kepler-10b with particulary high precision.
The astronomers think that there may be another planet orbiting this star, but they couldn’t confirm it yet.
Video: Kepler Mission Discovers Its First Rocky Planet. Source: NASA
A solar system with six planets: It is the most populated extrasolar system discovered yet. Published on the 3nd of February 2011 in Nature, it is the most impressive discovery made by the Kepler mission until now: a system with six planets, five of them seen transiting the star. Even a simultaneous transit of three of the planet was recorded.
The six planets are very close to its star, only 10% bigger than our Sun and located 2000 light years from us. The five inner planets are closer to its star than Mercury is to the Sun and the outer planet is half the way that Earth is. If we superpose this system over ours, all the six planets are inside the radius of the orbit of Venus.
Measuring the sizes and the masses of the five inner planets, the scientists have reached the conclusion that they are among the smallest exoplanets ever found, although compared to Earth they are relatively large for their masses. Their sizes vary from twice the Earth, for the smallest, to four times the Earth for the biggest. They are made of a mixture of rock and gas that may contain steam water. Kepler-11g, the outer planet, has an orbital period of 118 days, and the rest of the planes are between 10 and 47 days.
Scientists think that this kind of planetary system may be very rare, but they can’t say how rare it is because until now is the only one found.
The complexity of Kepler-11 and the information obtained from its study allowed astronomers to reach conclusions about the characteristics of its planets, and the study of its dynamic allowed to theorize about its formation. It has been observed that the planets Kepler-11d, Kepler-11e and Kepler-11f posses an important amount of light gases, which indicates that all of them were formed early in the system’s history, within a period of a few million years.
A planetary system is born when the core of a molecular cloud collapses to create a star. At that moment, discs of gas and dust are formed surrounding the star. This protoplanetary discs can be observed around the majority of stars less than a million years old, but there are few five million year-old stars still with them. This leads scientists to think that the planets that have a lot of gases are formed quickly, capturing the gas before the disc disperses.
Image: Artist’s conception of a simultaneous transit of three planets on Kepler-11. Source: NASA/Tim Pyle
The Kepler-11 system remains under study, and the chance of finding more planets is still open. They could be found observing their gravitational influence on the other planets, or by observation of new transits.
Video: Animation-Kepler-11 and Six Orbiting Planets. Source: NASA
Following this link you can see a chart summarizing the characteristics of all the planets found by Kepler: http://kepler.nasa.gov/Mission/discoveries/
A planet shower: The onslaught of discoveries unleashed by Kepler is only beginning. The past 2nd of February 2011, in addition to the publication of the discovery of the system Kepler-11, data on the stars identified as candidates to be harbouring planet candidates was released to the public: an impressive number of 1,235 planet candidates. Of these, 68 would have a size similar to Earth and there are 54 that are likely to be in the habitable zone where liquid water on the surface may exist. Kepler also found 288 planet candidates with sizes bigger than Earth, 662 with sizes similar to Neptune, 165 the size of Jupiter and 19 bigger than Jupiter.
Of the 54 planet candidates in the habitable zone, five are Earth-sized. The rest of them range in size from twice that of the Earth to bigger than Jupiter. These findings are based on data collected from 12th May to 17th September 2009 on the more than 156.000 stars in Kepler’s field of view, which is only one-four hundredth (1/400) of the sky.
These discoveries overturn some of the previous beliefs that astronomers had about the quantity and characteristics of planets in the galaxy. To begin with, they have shown that there are plenty of planets around the stars observed by Kepler. Also, they have shattered the theory that said that giant planets like Jupiter were more common than Neptune-size planets. The information revealed by Kepler points out to the hegemony of a smaller type of planet, with sizes ranging between Neptune’s and Earth’s.
Image: Kepler’s planet candidates by size. Source: NASA/Wendy Stenzel
Taking a look under the curtains of Science.
Eric B. Ford is Associate Professor in the Astronomy Department at the University of Florida, where he leads a research group studiying exoplanets. Eric, along with postdoc Dr. Althea Moorhead and two graduate students, Knicole Colón and Robert Morehead, belong to the scientific team that studies the data collected by the Kepler spacecraft. Thanks to him we have a chance to take a look inside the investigation.
Eric and his group collaborate in different tasks of the mission and are co-authors of several scientific papers presenting planet discoveries and other analyses. They have been working on various aspects such as testing the algorithms that analyze the light-curves of the stars, constructing computer models of the planetary systems to determine their properties, analysing the duration of the transits to study the shapes of planet’s orbits and the properties of their host stars and so on. Professor Ford is measuring the times of transits to look for small deviations from a perfectly periodic signal due to gravitational perturbations of other planets orbiting the same star.
Q: Why is the study of Exoplanets important?
A: No that long ago, all we knew about planets and how they formed was based on studies of our own solar system. We didn’t know whether it was very typical or highly unusual. Indeed, before astronomers began discovering other planetary systems in the 1990s, theories of planet formation had become so finely tuned that they attempted to explain many details of the solar systems. Yet once we began discovering planets very different than any in our solar system, it became obvious that we first needed to focus on much bigger questions about how our solar system was able to turn out the way it did. We’re still struggling to understand why different stars have such large differences in their planetary systems. Is it just a matter of chance? Or is there something about the ingredients or environment that heavily influences the final outcome?
Q: How did you get involved in this project?
A: Some of the Kepler team members have been working to make this happen since the 1980’s. In 2007, NASA issued a call for proposals for people to join the Kepler Science Team. I was one of nine scientists selected to join the mission.
Q: How does it feel working in a project like this, with so many people working on it? How is the work coordinated?
A: This is the first mission really designed from the ground-up to study exoplanets. It’s already made many first discoveries and we’re working hard so that Kepler will be able to make more. It’s a thrill to be a part of such historic project.
About coordination, merely operating a space telescope is a major task that takes a large team, let alone extracting the best possible data and turning that into science results. With so many team members, so many things to worry about, and so many discoveries to be made, it’s difficult to keep on top of things, to let others know of things you’re doing that could affect them, but not fill their inbox with tons of details that distract them from their important tasks. We use a lot of emails and telecons, plus wikis, websites, databases and occasional face-to-face team meetings to try to keep each other informed.
Q: So far, what is the discovery brought by Kepler that impressed you the most? Why?
A: The remarkable number of systems that appear to have multiple transiting planets.
Kepler-11 is the poster child, with its six planets, but there are many more that we’re hoping to confirm, too.
I and a few others were optimistic that Kepler might find some stars with two transiting planets. Some of our colleagues rejected our proposals to study such systems because they thought that they’d be so rare that there would be little to study. That’s the scientific equivalent of laughing at us. Even I didn’t expect to find a system with six transiting planets and so many more systems with multiple planet candidates.
Q: The Kepler spacecraft is in solar orbit: That means that the transit that Kepler detects cannot be detected from Earth?
A: No, many of the transits can be seen from both Kepler and ground-based observatories. The stars are so far away that the distance between Earth and Kepler is small potatoes.
Kepler is in solar orbit so it can observe the same field of stars virtually continuously. On Earth, we’d have to deal with the Sun and the Moon constantly interrupting our observations. Another important advantage of being in solar orbit is that it’s a much better environment for conducting precise measurements. If it were orbiting Earth, then there would be temperature changes due to the changing direction of the Sun, the shadow of the Earth, etc.
Q: How common planetary systems like Kepler-11are expected to be in our galaxy?
A: So far, we’ve confirmed only one such system, so it’s hard to say how common they are. On one hand, Kepler-11 seems to be an extreme planetary system in regards to having closely spaced planets. On the other and, we know that for every one we see, there are probably many more stars with similar planetary systems that are just oriented differently, so that we don’t fall in the shadows of their planets. And, of course, the galaxy contains over a thousand times as many stars as Kepler can search for planets.
Q: Which exoplanetary characteristics seem to be common and which seem to be rare?
A: Because it’s easier to find large planets close to their stars, we know of many such planets. However, that doesn’t mean that they’re necessarily the most common. On one hand finding so many weird planets suggests that there may be many more planets, perhaps including some like our own. On the other hand, each system where we find a close-in giant planet is one less star that could be similar to our own solar system. We’re still pushing to discover planets that have a star-planet separation similar to Earth and the Sun. And we’re still pushing to confirm planets the size of Earth. So we still don’t know if planetary systems like our own are common or rare.
Q: What has changed after Kepler about the ideas the scientists had about exoplanets?
A: We now know that planets with sizes between Earth and Neptune are quite common. Several theorists had predicted that such planets would be relatively rare.
Q: The big prize in the hunt for planets is to discover an Earth-like planet. Do you thing that Kepler will bring us “the new Earth”?
A: Right now there are several projects, each attempting to discover Earth-like planets. Kepler may or may not find the first Earth-size planet in the habitable zone. But I think there’s a very good chance that Kepler will find the first Earth-size planet in the habitable zone of a solar type star. For low-mass stars, the habitable zone corresponds to smaller orbital periods where other projects have a fighting chance. Of course, no one can promise what discoveries will be made in the future.
Q: What may change if a new Earth is discovered?
A: The primary goal of Kepler is determine the frequency of Earth-size planets and planets in and near the habitable zone. Unfortunately, in order to observe so many stars, Kepler looks at stars that are farther away and fainter than many other planet host stars we know of. Once we know how common they are, we can begin to make plans for future observatories that could find and study similar planets closer to Earth. If we get lucky, then it may be possible for future observatories like James Webb Space Telescope to begin to characterize the atmospheres of such planets.
Q: When are we going to get more discoveries from you?
A: We’re working on many things at once. Of course, the nature of research is that you don’t always know what you’ll find. The next major data release is scheduled for June 2012. So I suspect there may be another bog push to get our results around that time. There are also major meetings in September 2011, December 2011, and January 2012. Hopefully, we’ll have interesting new results for at least some of those.
Javier Barbuzano.
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