Whole New Worlds

What happens when you mix the music of Aladdin with astronomy? Something pretty wonderful:

The Halting of the Hot Jupiter

We haven’t talked about exoplanets for a while, and we should ‘cause they are pretty badass. Through various podcasts and the like, I've been hearing some really cool things about NASA’s Kepler Mission and all of neat astronomical bodies it’s been finding. So I decided to browse around the NASA and JPL websites to see what new coolness has been discovered recently.

NASA’s Kepler Mission was launched in 2009. It was built to detect potentially life-supporting planets around other stars. This satellite has a 0.95-meter diameter telescope that continuously and simultaneously monitors the brightness of 100,000 stars brighter than 14th magnitude in the constellations Cygnus and Lyrae. Kepler uses the transit method of planet finding, looking for the drop in the brightness of a star as a planet crosses in front of it at repeated, regular intervals. This dip in brightness not only tells us of the existence of a planet but also its size and orbit, from which we can calculate temperature. So far, the discovered extrasolar planets (or exoplanets) have been giant, mostly the size of Jupiter and bigger. A “hot Jupiter” is one of these large planets that orbits very close to its star. We’re talking less than 1 astronomical unit (AU), with orbital times of only 1-3 days! As such, they are really really hot. A new paper published in The Astrophysical Journal takes a look at one of the biggest questions in exoplanet research: how did these really big planets get so close to their stars? Or, perhaps more importantly, since they are so close, why weren't they pulled into their stars?

It is currently accepted that hot Jupiters formed further out from their host stars, likely beyond the snow line (or frost line), and then migrated in to their current, closer orbits. By observing how an exoplanet moves and how its parent star rotates (the “Rossiter–McLaughlin effect” for you space nerds), we also know that some exoplanets are misaligned and others are not. The misaligned exoplanets are likely directed inward due to interactions with other bodies in the system (gravitational scattering, the “Kozai mechanism”). Essentially, big things that have lots of gravity affect other big things that have lots of graving and they all push each other around. Aligned planets probably ended up where they are through migration in their primordial disk. As the conditions (density, temperature, magnetic fields, etc.) within this disk and the forming planet’s mass and density change over time the planet moves its orbit. Type-I migration assumes the density structure of the disk is affected by turbulence rather than by planets, and as such, is applicable to small mass planets. With Type-II migration, a gap between the planet and disk forms as the result of tidal torques from the planet becoming stronger than the viscous torques of the disk, and as such, is applicable to larger mass planets. The Type-II migration model is good at telling us how gas giants form beyond the snow line and move inwards but not so good at telling us how this migration stops once it is started.

The new study looks at the large ensemble of close-in exoplanets covering a wide range of host star (or stellar) masses in order to discern which mechanism halts exoplanet migration. First, the researchers collected exoplanet data and subdivided by mass the confirmed exoplanets located less than 1 AU from their star. Then they further subdivided Kepler candidates by estimated planet radius into three groups that approximated the terrestrial, super Earth/Neptune, and Jovian planet masses. Next, they attached stellar masses to the system, constraining their study to masses between 0.1 and 1.5 mass of the Sun.

The researchers then took this exoplanet data and put it into several “migration halting mechanism models” to see which model best explained the observations. Their goal was to “generate a reasonably simple prediction for the density of exoplanets as a function of stellar mass and semi-major axis within 0.1 AU.” Now, I’m not a modeler. I’m not even going to pretend I know how to put one together or even really describe it to you without confusing myself and you. Suffice it to say that the authors ran a bunch of models and did a bunch of Bayesian evaluations (my brain rebels at all things Bayesian too).

The astronomers found their tidal circularization model to provide the best mechanism for halting planet migration. Planet-planet scattering, secular chaos, and the Kozai cycle are all mechanisms that migrate planets inward and invoke tidal (or gravitational) forces on it. These forces lower the semi-major axis (the longest radii of an elliptical orbit) and eccentricity until the orbit of the planet becomes more circular. Put simply, when the gas giant gets close to its star, tidal forces cause the exoplanet’s elliptical orbit to become more circular. This circularization stabilizes the planet’s orbit, halting the inward migration and preventing the planet from getting eaten by its star. This result does not rule out the Type-II migration model, but instead, it says that it isn't the role of the primordial disk (or getting to the edge of it) to halt the migration.

If you will remember, they also took a look at stellar mass. They wanted to see if the mass of the star had an effect on the planets’ distance from it. Their models showed that halting distance depends on the stellar mass. This result actually provides further support to the favored tidal forces model. The tidal forces model predicts that hot Jupiters of more massive stars should, on average, orbit further out. Their results also show a halting distance-stellar mass dependence that was stronger than predicted, suggesting that future theoretical work may be needed to reproduce the observed exoplanet distributions.

This begs the question of why did our own Jupiter not migrate inwards? Or if it did, why did it stop so far away? We should definitely be glad that it did (or didn’t?) or our puny little Earth would have been eaten up or thrown out. Thank you Jupiter for ending up where you are.


Plavchan, P., & Bilinski, C. (2013). Stars Do Not Eat Their Young Migrating Planets: Empirical Constraints on Planet Migration Halting Mechanisms The Astrophysical Journal, 769 (2) DOI: 10.1088/0004-637X/769/2/86


Read JPL's story about this study: "Stars Don't Obliterate Their Planets (Very Often)"

Learn more about NASA's Kepler Mission: http://kepler.nasa.gov/
And I always find good stuff on the NASA and JPL websites!


(image via Cosmos, credit: ESA)

Exoplanet Extravaganza

This week I saw a couple of articles in Nature about new exoplanets. I'm generally interested in new anything but didn't stop to read the articles as astronomy jargon can sometimes make me feel like I couldn't understand a third grade solar system diorama. But I noticed these articles getting picked up by the news media around the world and so decided to check them out.

Exoplanets, or extrasolar planets, are planets that reside outside of our solar system and do not orbit around our Sun/star. In the search for extrasolar planets we are, in effect, searching for evidence that our solar system is not unique, that there are other planets out there that are similar to our own. As of February 1, 2011 there are 526 confirmed exoplanets and 1,235 that are waiting for confirmation. Of these, 68 are approximately Earth-size, 288 are super-Earth-size, 662 are Neptune-size, 165 are Jupiter-size, and 19 are super-Jupiter-size.

How do you detect a planet orbiting a very (and in astronomical terms that 'very' is oh-so-huge!) distant star? You can start by looking for the wobble. When you have two large bodies (such as planets and stars) they orbit a common center of gravity and gravitationally push and pull on each other, causing them to wobble back and forth. By measuring the wobble of stars you can gain all sorts of information, like planet size and mass, even though you can't actually see the planet. This gravitational push and pull can be utilized for planet hunting in other ways as well. Astronomers also use the Doppler shift to find planets. Basically, they are measuring the velocities of the stars by measuring the changes in the light that comes from a star moving towards us versus away from us. If the star is wobbling then the light emitting from that star will shift too. Scientists can also look for radial velocity changes, looking at transitions in atomic lines. As atoms transition between energy levels they either absorb or emit a photon in constant ways, add that up with an entire star and we can see bright and dark lines in the continuum of light. When a star moves it causes these lines to shift back and forth in color - toward us blue and away from us red.

Getting away from the wobble, there is also the planetary transit method. This is when the planet, as it passes in front of the star, blocks some of the light from that star. Now, planets are tiny compared to stars, so they don't block a lot of light but they can block enough to be detectable. Of course, you need to be looking at a solar system edge-on for this to work. There is also the gravitational micro-lensing technique. Gravity affects light, just like it affects just about everything else. So if you put a high-mass object between you and the light source (the star) then the gravity from that object will bend the light behind it into your field of view. This will make the objects appear to suddenly brighten as the object passes in front of the light source/star.

Recently new imaging technologies have become available. Technologies that will allow astronomers to take the first images of planets circling other stars. One such technology is coronagraphs. This was originally invented to study our Sun by blocking light coming from the solar disk in order to see the Sun's corona. This is being refined and adapted to find exoplanets, but it still has its problems (see Planet Quest). Another technique is using mirrors. Replace one large mirror with a lot of smaller mirrors and combine their light using a process called interferometry. This way you can have the small mirrors, which obtain a resolution equal to a single telescope as big as the largest separation between the individual telescopes, and gather information to build a larger picture.

The Kepler spacecraft was launched in March 2010 in order to explore the structure and diversity of planetary systems. To find extrasolar planets, particularly Earth-size terrestrial planets in habitable zones. It's speciality is the transit method and it observes 156,000 stars in the constellation Cygnus looking for those little blinks of light. The new papers in Nature reveal that the Kepler spacecraft has located a solar system with 6 known planets orbiting the star Kepler-11, 2000 light-years away. The planets are named Kepler-11b, Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f, and Kepler-11g, going from innermost to outermost. None of the planets is identified as Earth-like (bummer), but they are close, ranging from 2 to 4.5 times the radius of Earth. The planets are mostly mixtures of rock and gases, possibly including water, with the planets closest to the star having the highest densities and so likely having the most rock and water. The planets range in size between the masses of Earth and Uranus, with three being gas giants with thick hydrogen and helium atmospheres.

Alright, so why all of the excitement? First of all, a whole solar system discovered is always an exciting event, and this is the largest group of transiting planets orbiting a single star to be discovered. Kepler-9 has been found to have 3 transiting planets, and the star HD 10180 has at least 5 Neptune-like planets orbiting. Additionally, these planets are among the smallest found for which both mass and size have been measured. The Kepler-11 solar system's structure is also interesting and unique. Five of the six planets orbit very close to their star. I'm talking closer to their star than Mercury orbits to our star. The sixth planet orbits at about the distance of Venus to our Sun. But, as Kepler-11 is a smaller, cooler star than our Sun that means that these planets fall within the system's habitatable zone. This structure also means that it is a really densely packed system. It is unclear as to why the planets are so densely packed, but it may shed some light on planet formation and solar system evolution. It is pretty much accepted that planets form from a cloud of dust around a newly formed star. However, how and where they form within this protoplanetary disc is still debated. Do they form far from their parent star (at the distance of Jupiter and Saturn) and move inward over time? Or do they form in place? Kepler-11d, Kepler-11e, and Kepler-11f have a significant amount of light gas which indicates that they formed within a few million years of the system's formation. How do we know this? Well, we know that free hydrogen only lasts around 5 million years around a star before it gets dispersed by the solar wind. If these planets have a significant amount of hydrogen gas then they must have formed within 5 million years of the star igniting. The team suggests that "the small eccentricities and inclinations of all five inner planets imply...that gas and/or numerous bodies much less massive than the current planets were present," and that "the lack of strong orbital resonances argues against slow, convergent migration of the planets, which would lead to trapping in such configurations." For the planets for form in place it would require a massive protoplanetary disk of solids near the star and the accretion of large amounts of gas by hot small rocky cores, but the very high temperatures close to a growing star would have been too high for ices to have condensed. And so the solar system's evolution is still unclear. The closely packed nature of the Kepler-11 system does allow for lots of gravitational tugging and so good information for further study. It's a mystery, for now.

Now for lots and lots of read-it and look-it-up on your own links. Many of which have fantastic information, infographics, and links. Let's start with the actual paper:

Lissauer, Jack J., et al. (2011) A closely packed system of low-mass, low-density planets transiting Kepler-11. Nature: 470 (7332), 53. (DOI: 10.1038/nature09760)

And here is an editorial and article about extrasolar planets, also published in Nature.

The hunt is on for a distant planet similar to our own. Astronomers should decide just how similar it needs to be, before the candidates start pouring in. Nature: 470(7332), 5. (DOI: 10.1038/470005a)

Reich, Eugenie S. (2011) Beyond the Stars. Nature: 470(7332), 24-26. (DOI: 10.1038/470024a)

And here's a short video from NASA Ames Research Center about the Kepler Mission, exoplanets, and specifically the Kepler-11 system:



Here are also some lay-people-friendly articles from news outlets and the like about this story:
http://www.sciencedaily.com/releases/2011/02/110202133321.htm
http://news.sciencemag.org/sciencenow/2011/02/kepler-new-planets.html
http://www.nature.com/news/specials/exoplanets/index.html
http://www.nature.com/news/2011/110202/full/470027a.html
http://www.nature.com/news/2011/110202/full/470024a.html

Some information on exoplanets and how we find them:
http://www.astronomycast.com/astronomy/in-search-of-other-worlds/
http://planetquest.jpl.nasa.gov/technology/planet_imaging.cfm

Browse the Kepler Mission website: http://kepler.nasa.gov/

And explore the Planet Quest website from the NASA Jet Propulsion Laboratory CalTech:
http://planetquest.jpl.nasa.gov/

Become a planet hunter! Help find extrasolar planets from the comfort of your own living room (or wherever your computer lives). The citizen science project from the people behind Galaxy Zoo (www.galaxyzoo.org/) allows anyone with an Internet connection to contribute to astronomical research by sorting through data from NASA's Kepler mission. All you have to do is log in to the site and answer some simple questions about a star's brightness over time. If the star shows repetitive dimming then it is flagged as possibly having an exoplanet. Easy right? Check it out and contribute!
Planet Hunters: http://www.planethunters.org/
http://cs.astronomy.com/asycs/blogs/astronomy/archive/2010/12/20/become-a-planet-hunter.aspx
http://blogs.nationalgeographic.com/blogs/news/breakingorbit/2011/02/nasa-kepler-planet-hunters-new-data.html

(image from the above National Geographic source and courtesy of Tim Pyle, NASA)

A new hot ball of gas

It hasn't been until recently that astronomers have made great strides in locating planets outside our own solar system -- called exoplanets. The first exoplanets weren't found around normal stars but rather around pulsars (the remnants of dead massive stars). And that was in 1991! Since then astronomers have located hundreds of exoplanets - around 400 actually. A couple of techniques are considered common practice for locating exoplanets.

The Doppler shift technique looks at slight variations in the motions of stars - you can detect a planet tugging on its star or star wobble. A useful, if not very direct, technique. Another technique is to look for planetary transits. In this case the light of the star gets fainter (or is blocked/eclipsed) as the planet crosses in front of it (for more go to transitsearch.org). From these techniques you can get planetary characteristics such as size, mass, and orbit. Additionally, planetary spectroscopy (measuring the wavelengths of light passing through the atmosphere of a planet) can be used to identify various atmospheric attributes. Most of the planets that have been found so far fall within the "hot Jupiter" class. These are enormous gas giants - in many cases, several times the size of Jupiter - that orbit very close to their parent stars - at, or usually closer than, the orbit of Mercury. Needless to say these are very, very hot places.

Now the news: The discovery of another exoplanet has been reported in this week's edition of the journal Nature. This planet can be found in the CoRoT satellite, approximately 1,500 light years from Earth towards the constellation Serpens. The new planet was named Corot-9b (unofficial name Carrot Nimby....give me a second to giggle at that one). This planet regularly passes in front of its star - every 95 days. As with many others, this new exoplanet is a gas giant, it has a radius that is approximately 1.05 times that of Jupiter but is only 84% of its mass. Unlike others, it could have temperatures cool enough to host liquid water, lying between -23°C and 157°C. The planet orbits a sun-like star (similar but slightly cooler) at a distance similar to that of Mercury. It is suggested that the interior of this planet is close to that of Jupiter and Saturn, composed primarily of hydrogen and helium, but which may also contain up to 20 Earth masses of other elements such as water and rock.

Here's a couple of story links:
http://www.nature.com/nature/journal/v464/n7287/full/nature08856.html

http://www.eso.org/public/news/eso1011/

http://www.universetoday.com/2010/03/18/finally-a-normal-exoplanet/#more-60043