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Old 02 Feb 2018, 06:26 AM   #683384 / #1
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Default Exoplanet Stuff

We Might Need to Redefine 'Planet' Again
Looking at the high-mass end of planethood.

The upper limit is currently 13 Jupiter masses, which is where deuterium fusion begins. Above that limit is brown-dwarf stars, and the less-massive and older ones likely look outwardly much like Jovian planets.

Kevin Schlaufman proposes that formation mechanism is a better boundary. He notes that up to 4 MJup, planets' presence is correlated with the amount of "metals", elements heavier than hydrogen and helium. But above 10 MJup, that correlation is absent.

This implies that from 4 to 10 MJup is for H and He going from needing a pre-existing body to condense on to those elements not needing one.

Scientists Think They've Solved an Important Mystery About Brown Dwarfs

Their light curves have long showed mysterious behavior, and some people propose a solution: that they are much like planets like Neptune, with their continually-varying atmospheres.

New Observations Suggest Our Galaxy Contains 100 Billion Failed Stars -- brown dwarfs, between 13 and 80 Jupiter masses. Compare the 100 - 400 billion "real" stars that our Galaxy has.

Astronomers may have just discovered a new class of planets -- a Jovian planet that's almost a brown dwarf: ROXs 42Bb
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Old 02 Feb 2018, 06:44 AM   #683385 / #2
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A Giant Planet That's Freakishly Far From Its Sun
Canadian astronomers have discovered a gigantic planet-like object that's so far from its parent star that it takes 80,000 years to complete a single orbit — a distance that's 50 times farther than Pluto is from our sun. The discovery may force a re-think into how and where planets are capable of forming.

Called GU PSc b, it's a super-Jupiter that's 155 light-years away and about 10 times the mass of our Jupiter. It's 2,000 times farther from its star than the Earth is from the sun, or 67 times farther than Neptune. But according to the astronomers who made the discovery, the planet is bound to its star via gravity despite the extreme distance, telling the CBC that: "The planet is actually moving with its star."
Something like the Solar System's Oort Cloud.

A Star Came Within 0.8 Light-Years Of Our Sun 70,000 Years Ago
An international team of astronomers has identified a star that passed through the outer reaches of the Oort Cloud some 70,000 years ago. It came within a distance of 0.8 light-years, making it the closest known flyby of a star to the Solar System.

The star, dubbed Scholz's star, is actually part of a binary system. Its companion is a brown dwarf, a kind of "failed star" reminiscent of a gas giant. After analyzing its current trajectory, a group of astronomers from the United States, Chile, Europe, and South Africa have calculated that at its closest approach, it came within approximately 52,000 AU to the Sun, or 0.8 light-years. That's 5 trillion miles (8 trillion km), which, by cosmological standards, is excruciatingly close for an interstellar flyby of this nature.

NASA Scientists Have a Plan to Make Pluto a Planet Again
The proposal would redefine our definition of a planet in very simple terms. The scientists boil it all down to “round objects in space that are smaller than stars.” Yes, that would mean that Earth’s Moon, as well as many others, would be classified as a planet.

A more detailed description from the proposal breaks the method for planetary classification down as “a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”
That makes everything but the Sun in (Wikipedia)List of gravitationally rounded objects of the Solar System a planet.

  • Mercury
  • Venus
  • Earth -- Moon
  • Mars
  • Ceres
  • Jupiter -- Io, Europa, Ganymede, Callisto
  • Saturn -- Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus
  • Uranus -- Miranda, Ariel, Umbriel, Titania, Oberon
  • Neptune -- Triton
  • Pluto -- Charon
  • Haumea, Makemake, Eris, likely also Orcus, Ixion, 2002 MS4, Salacia, Varuna, 2005 UQ513, Quaoar, 2007 OR10, 2007 UK126, Sedna

The authors fault the IAU definition for implying that a planet can only orbit the Sun, and also for the problem of orbit clearing. With all its asteroids and comets, nothing in the Solar System would qualify as a planet.
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