A Wheel Pulled Into a Sliver
Take a bicycle wheel, perfectly round, and pull on opposite sides of the rim until it looks less like a circle and more like a long, narrow oval. Somewhere around a star roughly 278 light-years away, a planet several times more massive than Jupiter traces exactly that kind of path, and nothing in our own solar system is bent anywhere close to that far. Astronomers call it HD 80869 b, and NASA’s Exoplanet Archive lists its orbital eccentricity at 0.92, a plain, unitless ratio that measures how squashed an ellipse is: 0 means a circle, and the closer the number creeps toward 1, the longer and thinner the loop gets. Earth’s orbit sits at about 0.0167, close enough to a circle that no telescope operator would blink at the difference. HD 80869 b’s orbit is roughly 55 times more stretched than Earth’s.
A Ruler for Shape, Not Size
Eccentricity never tells you how far a planet actually travels, only what shape the loop is. The size of HD 80869 b’s loop comes from a separate number: its average distance from its star, about 2.91 astronomical units, nearly three times the gap between Earth and the Sun (one astronomical unit is that Earth-Sun distance, roughly 93 million miles). Stretch a loop that size to an eccentricity of 0.92, and the planet’s closest approach to its star and its farthest swing away from it end up enormously different, though 0.92 itself never states that difference in miles or kilometers. That’s a separate calculation the ratio alone can’t hand you.
How We Know
Nobody has photographed HD 80869 b, and with current instruments nobody will. It was found the way most giant planets far from their stars tend to be found: not by seeing the planet, but by watching its star flinch. As HD 80869 b swings around, its gravity tugs the star back and forth just enough to stretch and squeeze the starlight that reaches Earth, a method called radial velocity, where the star’s light shifts in a repeating pattern as it’s pulled toward and away from us. A team led by Demangeon and colleagues reported the discovery in 2021, describing HD 80869 b as one of six new “cold Jupiters” and calling its orbit one of the most eccentric on record; NASA’s Exoplanet Archive folded it into a batch it announced as “Seventeen New Planets!” on July 29, 2021. The 0.92 figure isn’t a dial reading, it’s what comes out when astronomers fit a model of an elliptical orbit to that wobble pattern. The velocity data is measured; the eccentricity is the model’s best match to it.
A Giant Built Like a Comet
A planet this heavy feels like it should move the way Jupiter does: slow, steady, nearly circular, the solar system’s biggest planet on its calmest path. NASA’s composite catalog puts HD 80869 b’s mass at roughly 1,610 Earth masses, with a radius estimated at about 1.14 to 1.15 times Jupiter’s, somewhere around 12.8 to 12.9 times Earth’s radius. But mass and orbit shape are separate questions. How stretched an orbit becomes usually comes down to history, not weight: a close gravitational encounter, a slow inward migration, a hidden companion star tugging from a distance. Big does not mean calm, and this planet sits squarely in that gap.
What’s Still Open
Two of NASA’s own catalog pages agree on an eccentricity of 0.92; a more precise figure sometimes quoted elsewhere isn’t confirmed in those entries, so 0.92 is the number to trust, and anything past the second decimal should be treated as unsettled. The mass is messier: NASA’s composite table puts HD 80869 b at roughly 1,610 Earth masses, which works out to about 5.07 Jupiter masses, but a separate catalog page from the same agency lists 5.66 Jupiter masses instead, a mismatch nobody has reconciled yet. And there’s no photograph of this planet to show you. Radial velocity hands astronomers a wobble curve, not a picture, so any image of HD 80869 b you come across is an artist’s concept, not a photograph, and it should say so.