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  • richardmitnick 10:33 am on March 20, 2017 Permalink | Reply
    Tags: , , , , , , Kepler-11, , , Sardines in Space   

    From astrobites: “Sardines in Space: The Intensely Densely-Packed Planets Orbiting Kepler-11” 

    Astrobites bloc

    Astrobites

    Title: A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11
    Authors: Jack J. Lissauer, Daniel C. Fabrycky, Eric B. Ford, et al.
    Lead Author’s Institution: NASA Ames Research Center, Moffett Field, CA, 94035, USA

    Status: Published in Nature 2011 [open access]

    The dawn of the Kepler Space Telescope data has unearthed a treasure trove of new and unusual celestial objects. Among these new discoveries is the planetary system Kepler-11. The system contains six transiting planets that are packed incredibly close around the Sun-like star, much like sardines are packed very closely in cans. The first five of these planets fall within the orbit of Mercury, and the sixth one falls well within the orbit of Venus. Few systems like this have been discovered; most planetary systems have a much larger separation between the planets, yet this system has its planets arranged in an extremely packed, yet extraordinarily still stable, way.

    1
    Figure 1: This figure from the NASA website is a visual representation of the Kepler-11 system, overlaid with the orbits of Mercury and Venus.

    When a single planet orbits a star, its period follows Kepler’s Laws to a tee; however, when other planets are introduced in the system, the orbiting bodies tend to perturb each other’s orbits. Their periods differ slightly according to the gravitational perturbations, and this variation is called a transit timing variation (TTV). Since Kepler-11 has five planets orbiting in extreme proximity to one another, it is the perfect illustration of measurements from transit-timing variations.


    Planet transit. NASA/Ames

    The photometric Kepler data marked the discovery of this system. The transits for each of the planets appeared separately in the light curve of the system. The light curve is just a measurement of the brightness of the star over time, so when a planet passes in front of the star, the brightness decreases, causing the dip in the light curve. The shape varies with each planet based on differences in size of the planet and orbital radius. From this data, it is possible to measure the radius of the transiting planet. This team followed up their photometric data with spectroscopic analysis from the Keck I telescope. This additional data allowed for the precise measurements of transit-timing variations, which yielded mass measurements for the inner five planets.

    For the first five planets, the TTVs were successfully measured, and with this information, the research team found the densities of the inner five planets, which yielded a surprising result. These planets, despite being densely packed, are not made of very dense material. Kepler-11b is both closest to the Sun and densest, but only with an overall density of 3.31 g/cm3. For comparison, Earth has an overall density of about 5.5 g/cm3. The densities of the planets orbiting Kepler-11 are depicted in Figure 2.

    2
    Figure 2: This shows the mass versus radius of the planets in the Kepler-11 system. The planets orbiting Kepler-11 are represented by the filled in circles. The other marking on the graph indicate planets in our solar system, shown for comparison. Figure 5 from today’s paper.

    While transit timing variations worked like a charm for the inner five planets, the sixth planet (Kepler-11g) was too distant from the others for this method to work well, so to confirm this planet, another method was employed. This team used several simulations to rule out alternate scenarios, which include chance alignment of the Kepler-11 system with and eclipsing star or with another star-planet system. This analysis successfully confirmed Kepler-11g , but because no TTVs could be measured for this particular planet, its mass and radius remain unknown.

    Even though this system has been more closely studied than most, the measurements have raised nearly as many questions as they have answered. The inner five have small inclinations and eccentricities, which implies some planetary migration process. However, since the periods of these planets are not in resonance, slow and convergent migration theories—which would naturally force the planets into resonant orbits—seem unlikely to be at play in this system. Formation of such a system is still a bit of a mystery. After all, such low-density planets are unusual and do not completely fit within the current understanding of planet formation.

    Kepler-11 continues to be one of the more intriguing planetary systems discovered, and its formation is not fully understood. Even though this system has been more closely studied than most, the measurements have raised nearly as many questions as they have answered. Systems like this extend our understanding of astrophysics, perhaps in a bit of an unexpected way; these closely packed planets have so much more to teach us about their system formation.

    See the full article here .

    Please help promote STEM in your local schools.

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    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 10:27 am on December 5, 2016 Permalink | Reply
    Tags: , , , Kepler-11, The Sun has a new twin and its children are weird   

    From astrobites: “The Sun has a new twin, and its children are weird…” 

    Astrobites bloc

    Astrobites

    Dec 5, 2016
    David Wilson

    Article: Kepler-11 is a Solar Twin: Revising the Masses and Radii of Benchmark Planets Via Precise Stellar Characterization
    Authors: Megan Bedell, Jacob L. Bean, Jorge Melendez et al.
    First author’s institution: Department of Astronomy and Astrophysics, University of Chicago, USA.
    Status: Submitted to AAS journals

    1
    Figure 1: Artist’s schematic of the Kepler-11 system, compared with our own. Six planets tightly orbit a star, which today’s paper reveals to be almost identical to the Sun. Image credit: NASA.

    The current tally of known exoplanets stands at around three thousand, depending on who you ask. Among the hundreds of stars that these planets orbit, Kepler-11 is a standout case (it even has its own Wikipedia page). Kepler-11 has six planets, five of which would comfortably fit inside the orbit of Mercury (Figure 1). What’s more, all of the planets have remarkably low densities for their size. The tight orbits and low densities are contradictory: the planets must have thick gas atmospheres, but the star should have blown away all of the gas that close before the planets could form. As the planets are there despite this, the system provides an excellent test bed for planetary formation models.

    Today’s paper presents new observations of the central star of the Kepler-11 system. As nearly everything we know about exoplanets comes from measuring their effects on their host stars, detailed observations of those stars are necessary to fully understand the planets. Due to its faintness, however, previous observations of Kepler-11 are poor.

    To remedy this, Bedell et al. turned to one of the largest telescopes in the world, the twin ten-metre Keck telescopes on Mauna Kea, Hawaii.

    Keck Observatory, Mauna Kea, Hawaii, USA
    Keck Observatory Interior
    Keck Observatory, Mauna Kea, Hawaii, USA

    Over two nights, Keck stared at Kepler-11, along with nine stars that the authors thought might be similar to it. The telescopes produced spectra, splitting the light of each star into a rainbow of colours, complete with distinct imprints left by the chemicals in the stars’ atmospheres. As a comparison, the authors also obtained a spectrum of the Sun, using the light reflecting off the dwarf planet Ceres to avoid burning out the telescope.

    2
    Figure 2: Spectrum of Kepler 11, the Sun and one of the comparison targets. The bottom plot shows the differences between the spectra of the two stars and the Sun. In both plots, the near-match of Kepler-11 and the Sun can be clearly seen. Figure 1 from Bedell et al.

    The results showed that Kepler-11, despite having a planetary system utterly unlike the Solar system, is nearly identical to the Sun. Careful analysis of the spectrum showed that Kepler-11 and the Sun have almost exactly the same temperature, mass, and atmospheric chemistry. Bedell et al. point out that, unusually for spectra, the result can be seen by eye. Figure 2 shows the spectra of Kepler-11, the Sun and a comparison star, with Kepler-11’s line nearly tracing over the Solar spectrum.

    Kepler-11 is therefore placed firmly in the category of star known as “solar twin”. It isn’t quite an identical twin, being slightly younger and with a slightly more metal-rich atmosphere, but it’s very close. These twins are extremely useful for exoplanet studies, as comparing their planetary systems to our own can guide how we apply the lessons learnt about the planets here to more distant worlds.

    With that in mind, where do these new observations leave Kepler-11’s intriguing planetary system? As the mass and radii of the planets are measured in relation to the star, the new results for Kepler-11 mean that the planets’ characteristics also need revising.

    The authors find that the planets all must have higher masses than previously thought, as well as smaller radii. This means that the densities of the planets are about 30 percent higher than previously thought. They therefore need less gas to form, making their formation so close to the star a bit more believable. With the deluge of planet discoveries set to continue for years to come, Bedell et al. finish by pointing out the need for similar, high-quality studies of the stars of all of these new worlds.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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