The HU Aqr system

HU Aqr is a star in Aquarius, 626.9 ly away, with 2 known planets. Their orbits run from 3.60 to 5.40 AU. All of them were detected by eclipse timing variations.

HU Aqr AB b · SOMEWHERE ON THIS ORBITHU Aqr AB c · SOMEWHERE ON THIS ORBIT2.0 AU
31 AUG 2026 10:28 UTC
Seen from above the orbital plane, on a linear scale — the star is at a focus of every ellipse rather than in the middle of it, and on any other radial scale these would stop being ellipses. This is not the view from Earth: from here the system is edge-on, which for most of them is how they were found. The planets turn anticlockwise. One orbit is drawn as an ellipse, from a measured eccentricity and a published argument of periastron. One is drawn as a dashed circle because the archive publishes an eccentricity of exactly zero with no uncertainty around it — a fit that held the orbit circular rather than a measurement that found it so. One orbit is drawn without a planet on it because nothing dates it: a period says how long a lap takes and not when one began. One orbit is dated and carries no planet anyway: its epoch is far enough back that the published uncertainty in its period has accumulated past a quarter of a turn, so which part of the orbit the planet is on is not known.
HU Aqr AB b3.60HU Aqr AB c5.40HU AQR
Distances are semi-major axes in astronomical units on a linear axis. Planet discs are on their own scale, by cube root of radius, because at the orbital scale every one of them is smaller than a pixel.
[

THE PLANETS

] 2, INNERMOST FIRST
PLANET
ORBIT (AU)
PERIOD
RADIUS
MASS
EQUILIBRIUM T
AB b
3.60
6.543 yr
12.80 R⊕
1880 M⊕
—
AB c
5.40
11.96 yr
12.90 R⊕
1430 M⊕
—
[

WHAT THIS PAGE IS

] AND IS NOT

Every figure here is the NASA Exoplanet Archive’s composite parameter for that planet — one consistent set per world, not the best number for each column from a different paper. Semi-major axes are orbits about the star, not distances from Earth; the system as a whole is 626.9 ly away.

No habitable zone is drawn: it needs the star’s luminosity, and neither a luminosity nor the radius and temperature that would give one is recorded for HU Aqr.

Discovery order and orbital order are different things. Planets are lettered b, c, d in the order they were found, and this table is ordered by semi-major axis instead — which is why the letters on many of these pages are out of sequence.

The plan view at the top puts the planets where their published elements say they are, and there is one thing it deliberately cannot show: which way the whole system is tilted. The inclinations the archive publishes are measured against the plane of the sky, one per planet, and there is no longitude of the ascending node for any planet in this catalogue — so the view perpendicular to the orbits is the only one that needs nothing invented. What that costs is the view from Earth, which for most of these systems is very nearly edge-on, because a system seen any other way does not transit and would never have been found.