Transiting exoplanet with longest known year: 704 Earth days

Jul 22, 2014

By Science Daily

Astronomers have discovered a transiting exoplanet with the longest known year. Kepler-421b circles its star once every 704 days. In comparison, Mars orbits our Sun once every 780 days. Most of the 1,800-plus exoplanets discovered to date are much closer to their stars and have much shorter orbital periods.

“Finding Kepler-421b was a stroke of luck,” says lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA). “The farther a planet is from its star, the less likely it is to transit the star from Earth’s point of view. It has to line up just right.”

Kepler-421b orbits an orange, type K star that is cooler and dimmer than our Sun. It circles the star at a distance of about 110 million miles. As a result, this Uranus-sized planet is chilled to a temperature of -135° Fahrenheit.

As the name implies, Kepler-421b was discovered using data from NASA’s Kepler spacecraft. Kepler was uniquely suited to make this discovery. The spacecraft stared at the same patch of sky for 4 years, watching for stars that dim as planets cross in front of them. No other existing or planned mission shows such long-term, dedicated focus. Despite its patience, Kepler only detected two transits of Kepler-421b due to that world’s extremely long orbital period.

The planet’s orbit places it beyond the “snow line” — the dividing line between rocky and gas planets. Outside of the snow line, water condenses into ice grains that stick together to build gas giant planets.

“The snow line is a crucial distance in planet formation theory. We think all gas giants must have formed beyond this distance,” explains Kipping.

4 comments on “Transiting exoplanet with longest known year: 704 Earth days

  • “Finding Kepler-421b was a stroke of luck,” says lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA). “The farther a planet is from its star, the less likely it is to transit the star from Earth’s point of view. It has to line up just right.”

    The more distant from their star the harder they are to find and the longer the time taken for an orbit to come back to eclipse their star again in a new transit.

    NASA’s Kepler spacecraft is doing a good job of finding planets orbiting further out from their stars.

    In the Solar-System for example, Neptune revolves around the sun once every 165 years, which means that an extra-Solar-System observer would have to wait about 165 years between transits, even if they were lined up on the plain of the ecliptic!



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  • Reminds me of a SciFi trilogy called Helleconia Spring, etc.. The premise was a planet that orbited its sun in a highly eccentric elliptical orbit with a 3000 year period. The “Humans” would discover relics and buildings from previous civilizations that grew during the spring period but died out during the extremely hot summer, with remnants surviving through Autumn rebuilding, then getting decimated again in winter. Every rotation of the planet created new civilizations. It was a great read.

    http://en.wikipedia.org/wiki/Helliconia

    It appears planets nearly always form as a by product of star formation. The leftover stuff coalesces into planets, some gas, some solid. What I try to ponder, without much success is how many planets are out their in the universe, given that just about every star has the potential to produce planets. Numbers so huge, that I cannot comprehend. I wonder if their is enough mass to contribute to the equations governing expansion or collapse of the universe.

    Shades of Asimov’s Foundation series. The universe must be teeming with life.

    Another anecdote. I an remember as a young child, when my mom was doing the washing, we had a sink that would collect the soapy water. I would wait till the surface was perfectly still. I would very gently pull out the plug via the chain, then watch vortex that formed as the water went down the sink hole. The suds would slowly start to spin, form spiral arms and look just like a galaxy with a black hole at the centre. I guess I was always going to be a godless skeptic.



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  • I am reading about the snow line now. I thought they were rethinking that since many hot, large gas planets were being found within the snow line.

    ” Since gas giant planets can be found extremely close to their stars, in orbits lasting days or even hours, theorists believe that many exoplanets migrate inward early in their history. ”

    Really? How do they do that?



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  • Neodarwinian Jul 22, 2014 at 8:45 pm

    I am reading about the snow line now. I thought they were rethinking that since many hot, large gas planets were being found within the snow line.

    No. The hot gas giants are being found near to their stars, although large planets can be generating some internal heat from radioactive decay.

    There are two reasons why gas giants don’t form and remain in an inner solar system.

    There is less matter to collect in a shorter orbit.
    Solar radiation and solar winds blast volatile elements off them and carry the volatiles to the outer system: (Perhaps it should be called the “Fry Line”?) – leaving a rocky core. If there are gas giants near stars they are probably in the early stages of evolution after their arrival.

    ” Since gas giant planets can be found extremely close to their stars, in orbits lasting days or even hours, theorists believe that many exoplanets migrate inward early in their history. ”

    Really? How do they do that?

    In settling into more circular orbits planets interact with each other at resonant frequencies, which can make orbits more elliptical or eccentric, causing collisions or further gravitational interactions.

    As you may know because of tidal drag reducing spin, and conservation of angular momentum, our Moon has been, and is gradually moving into a higher orbit. The same effect happens on planets interacting with their stars.

    If the higher orbit causes the planet (or moon) to intrude on the orbit of the next most distant planet (or moon), they can collide or slingshot around each other into new orbits, – nearer or more distant from their star (or planet).
    It is thought that this happened to the gas giants of the Solar System. http://astroclock2010.wordpress.com/cosmic-timeline-17/

    Sometime they settle into regular resonantly synchronised orbits, as with Jupiter’s moons. https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Orbital_resonance.html



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