HD 80606 b

A gas giant

11.57 Earth radii, Jupiter-class (Jupiter is 11.2).

The orbit

Its year takes 111 Earth days (0.3 Earth years). The orbit is strongly stretched (eccentricity 0.93), so its distance from the star, and with it the climate, swings dramatically.

Temperature and starlight

Its equilibrium temperature (the airless-world estimate) is about 132 °C. Above water's boiling point. It receives 4.7 times the starlight Earth gets.

Its sun

Its star is Sun-like (5,565 K, G5; the Sun is 5,772 K). The system holds 2 stars, so its skies have more than one sun.

How far is that, really?

At 217 light-years, tonight's light from this system left it around the year 1809. A Voyager-speed probe would need roughly 3,835,749 years to get there.

How we found it

It was found by the radial-velocity method: the planet's gravity swings its star in a small circle, and that wobble shows up as a rhythmic Doppler shift in the starlight. The wobble's size gives the planet's minimum mass. The discovery was announced in 2001 from Multiple Observatories.

Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.

01 A World Discovered by Radial Velocity

The story of HD 80606 b's discovery begins with a subtle wobble. In 1999, the G-Dwarf Planet Search — a survey targeting nearly 1,000 nearby G-type stars — noticed that HD 80606 showed a variable radial velocity when observed with the 10-meter Keck 1 telescope at the W. M. Keck Observatory in Hawaii. That variation hinted at an unseen companion tugging the star back and forth. The Geneva Extrasolar Planet Search team then took over, using the ELODIE spectrograph mounted on the 1.93-meter telescope at the Haute-Provence Observatory in France to characterize the signal precisely. The formal announcement came on 4 April 2001, led by Michel Mayor and Didier Queloz — the same pair who had discovered the first confirmed hot Jupiter, 51 Pegasi b, years earlier. The detection of the transit itself came through a far more modest instrument: a Celestron 35-centimeter Schmidt–Cassegrain telescope at the UCL Observatory, demonstrating that world-class science does not always require a world-class telescope.

02 An Orbit That Rivals Halley's Comet

HD 80606 b holds one of the most extreme orbits ever measured for a planet, with an eccentricity of approximately 0.9318 — a figure comparable to that of Halley's Comet. For context, Earth's eccentricity is only about 0.017, a shape so close to circular it is barely distinguishable from one. HD 80606 b, by contrast, follows a dramatically elongated path that carries it from just 0.03 AU from its star at closest approach (periastron) out to 0.88 AU at its farthest point (apastron) — a swing of nearly the full distance between the Earth and the Sun. The entire orbit completes in 111 days. At apastron, the planet receives roughly the same stellar energy as Earth receives from the Sun. At periastron, the insolation rockets to around 800 times greater than what Mercury endures in our own Solar System. The planet currently ranks as the most eccentric known planet after HD 20782 b, placing it in genuinely rare astronomical company.

03 The Kozai Mechanism and a Tilted World Deeper

Scientists have a compelling hypothesis for why HD 80606 b's orbit is so outrageously elongated: the Kozai mechanism. This gravitational process can occur when a planet's orbital plane is significantly tilted relative to the plane of a binary star system. HD 80606 is itself part of a binary, and if the planet's orbit is sufficiently inclined to the stars' mutual orbit, gravitational perturbations from the companion star can pump up the planet's eccentricity over time. Observational support for this picture comes from measurements of the Rossiter–McLaughlin effect, a subtle distortion in starlight that occurs as a transiting planet blocks different rotating portions of the stellar disk. Those measurements indicate the planet's orbit is tilted by 42±8° relative to the rotational axis of the host star — exactly the sort of misalignment one would expect if the Kozai mechanism were reshaping the orbit. The interpretation remains consistent with the evidence, though pinning down the full geometry of the binary system's influence is an ongoing challenge.

04 Temperature Spikes of a Periastron Passage

In 2009, astronomers achieved a remarkable observational feat: they detected the secondary eclipse of HD 80606 b — the moment when the planet passes behind its parent star — and used it to probe the planet's temperature as it swung through periastron. The results were dramatic. The planet's temperature climbed from around 800 K (527 °C; 980 °F) to 1,500 K (1,230 °C; 2,240 °F) in just 6 hours. That is a rise of 700 K in about a quarter of a day — a heating rate with no real analogue in our Solar System. The secondary eclipse technique works because when the planet disappears behind the star, the combined light of the system drops by the amount the planet was contributing; by measuring that drop at infrared wavelengths, scientists can infer how much heat the planet is radiating. These measurements turned HD 80606 b from an abstract orbital curiosity into a laboratory for studying extreme atmospheric heating events in real time.

05 Shock Wave Storms at Five Kilometers Per Second

The wild temperature swings experienced by HD 80606 b do not pass through the atmosphere quietly. Computer models predict that the planet heats up by 555 K (280 °C; 540 °F) in just hours as it rounds periastron, and that pulse of energy triggers what researchers describe as shock wave storms. These storms radiate outward from the sub-stellar point — the face of the planet most directly aimed at the star — and sweep across the gas giant's atmosphere. The winds associated with these events are predicted to reach speeds of around 5 kilometres per second, which translates to roughly 11,000 miles per hour or about 3.1 miles per second. For comparison, the fastest sustained winds ever measured on Earth topped out near 113 metres per second during a tropical cyclone — less than one fortieth of the speeds these models suggest on HD 80606 b. Whether the models capture the full complexity of such rapid, large-amplitude forcing is an open question that future observations may help resolve.

06 How Large is the Planet, and How Dense?

Because HD 80606 b transits its host star — passing directly across the stellar disk as seen from Earth — its physical radius can be measured using the transit method, which looks at how much starlight the planet blocks. Those measurements reveal a radius approximately equal to Jupiter's, despite the planet carrying a mass roughly 4 times greater than Jupiter. Packing 4 Jupiter masses into roughly 1 Jupiter radius produces a planet that is substantially denser than Jupiter itself, yet the article notes its density is slightly less than Earth's — which at 5,514 kilograms per cubic meter is considerably denser than typical gas giants. This places HD 80606 b in a regime where gravity compresses the deep interior significantly. The transit also takes a notably long time to complete: the planet spends 12.1 hours crossing the face of its star, a lengthy event that reflects both the planet's large size and the geometry of its orbital path at the point of transit.

07 A Sky Unlike Any Other: The View From the Cloud Tops

Imagining HD 80606 b from within its own atmosphere offers one of the most visually arresting thought experiments in planetary science. An observer hovering above the cloud tops would watch the host star swell as the planet rushes in from apastron toward periastron. At closest approach, the star would appear 30 times larger in the sky than the Sun appears from Earth. The Sun as seen from Earth already looks large enough to cause sunburn and power photosynthesis across an entire biosphere; an object 30 times wider in angular diameter would subtend 900 times the solid angle and would dominate the sky in a way that has no meaningful human analogue. Then, within hours, the heating spike would arrive — the atmosphere lurching from 800 K to 1,500 K — before the planet swings back outward and the star shrinks again over the following weeks. At apastron the insolation would drop to roughly Earth-like levels, a gentle interlude before the next violent periastron passage 111 days later.

08 A Record-Holder Before Kepler Changed Everything Deeper

Before the Kepler Space Telescope's large data release in February 2011, HD 80606 b held the record for the longest orbital period of any known transiting exoplanet. An orbital period of 111 days was and remains exceptional for a transiting world; most transiting planets discovered by ground-based surveys have periods of only a few days, because shorter periods mean more frequent transits and a higher geometric probability of the orbit being aligned favorably with the observer's line of sight. HD 80606 b's transit geometry is accordingly rare. The transit of 14 January 2010 attracted observational attention and was partially captured by the MOST satellite, though equipment failures disrupted coverage of part of that event, and the secondary transit of 8 January was lost entirely. The next predicted midpoint after that was 1 February 2013 at 11:37 UT, illustrating just how infrequently opportunities to observe this planet arise and how precious each successful observation window becomes for the scientific community.

09 Rotation That Defies Simple Tidal Theory Deeper

For planets on tight orbits, tidal forces from the host star tend to synchronize the planet's spin to its orbital motion — a process called tidal locking. For highly eccentric orbits, theorists predict an intermediate state called pseudo-synchronous rotation, where the planet's spin rate matches the orbital speed near periastron rather than averaging over the full orbit. For HD 80606 b, the predicted pseudo-synchronous rotation period is 40 hours. The measured rotation period, however, is 93 hours, with a substantial uncertainty ranging from +85 to −35 hours. That range is wide enough to be consistent with many scenarios, but the central value is notably longer than the pseudo-synchronous prediction — more than twice as long. This discrepancy suggests either that tidal evolution has not yet driven the planet to its equilibrium spin state, that the theoretical models of pseudo-synchronization need refinement for such extreme eccentricities, or that the measurement itself carries systematic uncertainties not fully captured in the quoted error bars. It remains an open and interesting problem.

10 Where in the Sky to Find It

HD 80606 b resides in the constellation Ursa Major, the Great Bear — one of the most recognizable patterns in the northern sky and home to the famous Big Dipper asterism. The host star HD 80606 lies approximately 217 light-years from the Sun, a distance close enough that the star is accessible to moderate telescopes but far enough that the planet itself is of course invisible to any amateur instrument. The star is also catalogued as HIP 45982, giving it an entry in the Hipparcos astrometric catalog, and the system carries the alternative designation Struve 1341 Bb, reflecting its membership in a binary star pair. For observers hoping to catch a transit, the 111-day orbital period means opportunities are rare and each one deserves careful advance planning. With a transit duration of 12.1 hours, a successful observation demands a long, clear night — and a degree of patience and logistical coordination appropriate to one of the solar neighborhood's most extreme planetary systems.

11 A Giant in a Binary Star System Deeper

HD 80606 b does not orbit its star in isolation from other large bodies. The host star HD 80606 is itself a member of a binary star system, and this stellar companion is likely the key to understanding why the planet's orbit is so dramatically eccentric. The Kozai mechanism, invoked to explain the orbital shape, requires exactly this kind of architecture: a distant stellar companion whose gravitational influence periodically exchanges the orbital inclination of an inner planet for eccentricity, or vice versa. Over millions or billions of years, such interactions can drive a planet that formed on a nearly circular orbit into the kind of comet-like trajectory HD 80606 b now follows. The Rossiter–McLaughlin measurement of a 42±8° spin-orbit misalignment adds a further layer of evidence that the planet's current configuration has been sculpted by forces beyond its immediate host star. The binary companion is not just background scenery — it may be the dominant force that has shaped this planetary system into such an unusual state.

About this record

The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.