:INFO
Gas giant around Kepler-391
It is 2.88 times Earth's radius, closest in size to Neptune, and 450.0 times Earth's mass, at 400.00 g/cm3, denser than Earth. A year there lasts 20.50 days, on an orbit 2 times closer to its star than Mercury is to the Sun. Its equilibrium temperature is 662 K, hotter than any desert. The host star Kepler-391 runs at 5038 K with 1.27 times the Sun's mass.

:COUNTER
400.0 g/cm3: denser than iron at Earth surface pressure

:STATS
[icon:TARGET] Radius | 2.88 Earth radii
[icon:CHART] Mass | 450.0 Earth masses
[icon:FIRE] Equilibrium temperature | 662 K (measured)
[icon:CLOCK] One year | 20.50 days
[icon:BOLT] Orbit radius | 0.1640 AU
[icon:STAR] Host star | Kepler-391 at 5038 K
[icon:USERS] Distance | 2835 light-years
[icon:TRENDING_DOWN] Starlight blocked | 0.008% at each transit

:PATH
What you get from the shadow
Block a known fraction of a known star and you have measured a width.

:NOTE
A transit gives the radius and a separate measurement gives the mass. Having both is what makes a density, and a density is the only clue to composition at this range.

:NOTE
Found by Kepler in 2014. Kepler's method needed repetition, not brightness. It could find small planets around faint stars, provided they came round quickly enough to be seen twice.

:NOTE.half
Kepler-391 has 2 known planets. Systems like this are the ones where architecture can be studied instead of guessed at.

:NOTE.half
Its orbit is noticeably elliptical, eccentricity 0.27, so the starlight it receives changes substantially over one year.

:LINK
https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-391%20c
NASA Exoplanet Archive: source data, including the values not used here