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    Administrator lpetrich's Avatar
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    Exoplanet Stuff

    Simulations explain giant exoplanets with eccentric, close-in orbits -- ScienceDaily
    As planetary systems evolve, gravitational interactions between planets can fling some of them into eccentric elliptical orbits around the host star. Smaller planets should be more susceptible to this gravitational scattering, yet many gas giant exoplanets have been observed with eccentric orbits. In fact, the planets with the highest masses tend to be those with the most eccentric orbits. A new study explains these counter-intuitive observations.
    Signatures of a Planet–Planet Impacts Phase in Exoplanetary Systems Hosting Giant Planets - IOPscience
    Exoplanetary systems host giant planets on substantially noncircular, close-in orbits. We propose that these eccentricities arise in a phase of giant impacts, analogous to the final stage of solar system assembly that formed Earth's Moon. In this scenario, the planets scatter each other and collide, with corresponding mass growth as they merge. We numerically integrate an ensemble of systems with varying total planet mass, allowing for collisional growth, to show that (1) the high-eccentricity giants observed today may have formed preferentially in systems of higher initial total planet mass, and (2) the upper bound on the observed giant planet eccentricity distribution is consistent with planet–planet scattering. We predict that mergers will produce a population of high-mass giant planets between 1 and 8 au from their stars.
    NASA Exoplanet Archive - ICE Plotter

    That oddity is real. Of known exoplanets, their ranges of orbit eccentricities increase with increasing mass, with the champion having a projected mass of 1.5 Jupiter masses and an eccentricity of 0.95. That means that its maximum distance is 40 times its minimum distance. Some more details on that champion:

    It is HD 20782 b, and it is the only known planet in its system. Its period is 1.635 (Earth) years, and its semimajor axis 1.36 AU. Its distance from its star thus ranges between 0.068 AU and 2.652 AU. The time it spends at less than twice its minimum distance is 4 days, and less than its s.m. axis distance 118 days or 0.323 years.

    The star is HD 20782, and its mass is 1.07 solar masses, its radius 1.11 solar radii, its effective temperature 5790 K, and its spectral type G3V. Its metallicity index is -0.06 dex for Fe/H. Its luminosity I calculate to be 1.25 solar luminosities.

    HD 20782 is almost a dead ringer for the Sun. Much like 51 Pegasi, the first "normal" star to be discovered to have an exoplanet.

    The planet's equilibrium temperature thus ranges between 170 K and 1100 K.

    The maximum period of that planet's moons, if any, is about 6.7 days (orbit period for the planet having a circular orbit at its closest distance). So the planet is not likely to have many moons.

    The system is 36 parsecs / 117 light years away, and the star's apparent magnitude is +7.4. Its coordinates are RA = 03h20m03.58s Dec = -28d51m14.7s or RA = 50.014908, Dec = -28.854071.

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    Administrator lpetrich's Avatar
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    The paper's authors propose that more than one Jovian planet can form, and that they can sideswipe each other or collide with each other, making the remaining planet(s) have high orbit eccentricity.

    This brings to mind an oddity called the "Kepler dichotomy" - between single-planet systems and multiplanet systems. A big problem here is that transit detection requires a line-of-sight coincidence, and the method will thus be unable to detect the great majority of planets. It will even fail to detect many planets in systems where some planets could be observed. But there is a way out of that, to extrapolate to single planets from multiplanet observations. One finds something like twice as many single planets than what one might expect. Furthermore, the single planets' stars tend to have higher metallicity than average, giving support to the OP's scenario of planets too massive to easily coexist.

    When Exoplanets Collide | NASA - evidence of collision of rocky planets in the form of interplanetary dust.


    First exoplanet found around a Sun-like star (Nature magazine) - 51 Pegasi b, a hot Jovian planet, and the first of this weird kind of planet ever discovered.

    Lessons from scorching hot weirdo-planets - "While these close-in, hefty worlds represent about 10 percent of the exoplanets thus far detected, it’s thought they account for just 1 percent of all planets." Because they are the easiest planets to detect.

    Rebekah Dawson notes three theories about "hot Jupiters":
    • They formed close in
    • They formed farther out, then spiraled in by interacting with the protoplanetary nebula
    • They got kicked into very eccentric orbits, then their orbits got circularized by tidal friction caused by their stars

    Another big trend is that hot Jupiters tend to be around stars that are more metal-rich. Astronomers refer to metals as any element heavier than hydrogen or helium. There’s more iron and other elements in the star, and we think that this may affect the disk of gas and dust that the planets formed out of. There are more solids available, and that could facilitate forming giant planets by providing material for their cores, which would then accrete gas and become gas giants.

    Having more metals in the system could enable the creation of multiple giant planets. That could cause the type of gravitational interaction that would put the hot Jupiter onto a high eccentricity orbit.

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    Contributor repoman's Avatar
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    Speaking of Jovian planets, just found out about Simon Marius today who found the Galilean satellites a smidge before Galileo himself and named them (suggested by Kepler?)

    https://en.wikipedia.org/wiki/Simon_Marius
    Marian moons?

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    Veteran Member Treedbear's Avatar
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    Quote Originally Posted by repoman View Post
    Speaking of Jovian planets, just found out about Simon Marius today who found the Galilean satellites a smidge before Galileo himself and named them (suggested by Kepler?)

    https://en.wikipedia.org/wiki/Simon_Marius
    Marian moons?
    There's also this:
    He also concluded from his observations of the Galilean moons that they must orbit Jupiter while Jupiter orbits the Sun. Therefore, Marius concluded that the geocentric Tychonic system, in which the planets circle the Sun while the Sun circles the Earth, must be the correct world system, or model of the universe.
    So good that he got to name the moons but Galileo gets to claim them.

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    Administrator lpetrich's Avatar
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    Building planets, piece by piece - on the "pebble accretion" theory, that protoplanets collected pebble-sized chunks.
    Scientists used to think that planets formed as planetesimals collided and merged, like fistfuls of Play-Doh slapped together. But it turns out that that process would have taken too long. So astronomers recently proposed a new way to explain how baby planets grew.

    Computer simulations show that small pebbles within the dusty disk would have glommed onto the growing protoplanets. Those tiny pebbles coalesced so rapidly that the protoplanets grew quickly into full-fledged planets — like a kid suddenly packing on enough pounds to become an adult.

    These so-called 'super-puff' worlds could be exoplanets with ring | Astronomy.com - "Astronomers investigate whether mysterious low-density planets are actually ringed planets that have been misunderstood."
    Shreyas Vissapragada, a planetary astronomer at Caltech, started thinking about the possibility of super-puffs being ringed planets after another astronomer asked him about it.

    “If an alien observed Saturn with the Kepler space telescope, how badly would they get the density wrong if they didn't realize it had rings?” Vissapragada said he asked himself. He did the math and found that they might calculate Saturn’s density to be only half of what it really is.

    He teamed up with Anthony Piro, an astronomer at the Carnegie Institution for Science, to investigate what this could mean for Earth astronomers’ observations.

    The pair considered what the ring systems would have to be like for known super-puffs to be ringed planets. For example, most known super-puffs are fairly close to their stars, so their rings would have to be rocky rather than icy. Some of the planets wouldn’t be able to have rocky rings wide enough to throw off density estimates, because rocky material that's too far from the planet would clump to form moons. For other super-puffs, rings might still be a possibility.
    The article had a picture of a simulated Saturn transit -- at maximum radial tilt, the planet's rings cover up as much of the Sun as the planet itself.

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    Administrator lpetrich's Avatar
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    NASAExoplanetArchive (@NASAExoArchive) / Twitter
    NASA Exoplanet Archive
    • Confirmed Planets: 4104
    • TESS Confirmed Planets: 37
    • TESS Candidates: 1517


    Some exoplanet researchers:
    Sara Seager (@ProfSaraSeager) / Twitter
    Natalie Batalha (@nbatalha) / Twitter
    Sarah Ballard (@hubbahubble) / Twitter

    Amateur astronomers can get in on the fun, though they have to have some high-quality equipment by amateur standards.
    Exoplanet Section | aavso.org
    Amateur Astronomers Join Hunt for Exoplanets

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