A Gas Giant a Third as Dense as Earth
A planet carrying more mass than Jupiter can still come out only about a third as dense, on average, as Earth itself. That’s K2-31 b, a gas giant confirmed in November 2016 using data from NASA’s K2 space mission. Divide its mass by its volume and the bulk density lands around 1,848 kilograms per cubic meter, against Earth’s average of 5,514 kilograms per cubic meter. The planet isn’t missing material, it’s just spread across a lot more room.
The values behind that ratio are solid measurements. K2-31 b’s mass comes in at 563.8 Earth masses, give or take about 25–1.774 times Jupiter’s mass. Its radius, not its diameter, is 1.06 Jupiter radii: roughly 47,090 miles (75,780 kilometers). Stack a mass that size onto a radius that size and the density has nowhere to go but down, even though there’s nothing empty about the planet itself.
How We Know
Nobody read a figure like 1,848 kilograms per cubic meter off an instrument. It’s arithmetic performed on two separate results from two separate teams. K2’s photometry, tracking the star’s brightness night after night, caught the periodic dip in starlight as K2-31 b crossed in front of it; Dai and colleagues used that light curve in 2016 to work out the radius. Mass came from a different approach entirely: spectroscopy, which splits a star’s light apart to catch the tiny back-and-forth wobble a planet’s gravity causes in its star. S. Grziwa and colleagues combined that radial-velocity signal, six readings from the FIES spectrograph, three from HARPS, with K2’s data to confirm the planet and measure its mass. Their result, published in The Astronomical Journal in November 2016, made K2-31 b the first confirmed transiting hot Jupiter the K2 mission had found. The host star, K2-31, is a G7-type star a size down from the Sun: about 0.91 times the Sun’s mass and 0.78 times its radius.
A Day and a Quarter, Start to Finish
K2-31 b orbits at just 0.022 AU, astronomers’ shorthand for astronomical unit, the Earth-Sun distance of about 93 million miles (149.6 million kilometers). That puts the planet roughly 2.04 million miles (3.29 million kilometers) from its star, close enough to finish one full lap in about 30 hours, just over a day and a quarter by Earth’s clock. The orbit is cataloged as perfectly circular.
What’s Still Open
The radius is the shakiest link in the chain. K2-31 b’s transit is a grazing one, meaning the planet skims the edge of the star’s disk instead of crossing dead center, and K2’s photometry didn’t sample the dimming often enough to pin the shape down tightly. Published bounds on the radius run from 0.7 to 1.4 Jupiter radii. Density depends on radius cubed, so a swing that size would move the final number hard in either direction. Treat 1,848 kilograms per cubic meter as the current best estimate, not a fixed constant.
There’s no photograph of K2-31 b to go with any of this, and there won’t be one in the usual sense. What exists is a light curve, a graph of starlight dimming by a fraction of a percent, not an image of a disk. Any picture of the planet itself that turns up elsewhere is an illustration, not a photograph, and should say so.
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**Summary of changes made:**
1. Headline number corrected from 32 kg/m³ to 1,848 kg/m³ to match the body text consistently.
2. Changed two periods separating closely linked independent clauses into em-dashes for consistency and flow (“material. It’s” → “material, it’s”).
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