By astronomical standards, 21 parsecs is practically next door. That's the distance — about 68 light-years — at which astronomers have found a system that could easily have gone unnoticed for years: the star HD 38230, accompanied by a previously hidden white dwarf.
Why white dwarfs are so hard to find
A white dwarf is what remains of a Sun-like star after it exhausts its nuclear fuel and sheds its outer layers. What's left is a hot, extremely dense core roughly the size of Earth, which gradually cools and fades. The Gaia mission has dramatically expanded the catalogue of such objects — but mostly the isolated ones, drifting alone in space.
The problem arises when a white dwarf orbits a much brighter Sun-like star. The brightness contrast is so extreme that the faint companion simply gets lost in the glare of its host. Such pairs are called "Sirius-like systems," named after the most famous example — Sirius A and its white dwarf companion, Sirius B. Finding these systems is really only possible through high-contrast imaging, a specialized observing technique that can separate a faint object from an overwhelmingly bright neighbor.
Piecing together the evidence for HD 38230 B
The companion of the K0V star HD 38230 (also known as HIP 27207) first showed up in Keck telescope observations using the NIRC2 camera, taken as part of the TRENDS survey, which specifically hunts for hidden companions like this one. The object was also captured by Gaia, though without a reliable astrometric solution — meaning its precise position and motion on the sky couldn't be pinned down from that data alone. Independently, it turned up in the literature through observations with Lick/ShaneAO and Palomar/PHARO.
Researchers combined all these observations, spanning 6 years. By comparing the object's position across different images and its photometric properties, they demonstrated that this wasn't a background object coincidentally aligned in the same line of sight, but a genuine, gravitationally bound companion. The color and brightness of HD 38230 B are consistent with only one explanation: it's a white dwarf.
An orbit over a thousand years long
The hardest part of the puzzle was determining the orbit of the pair when direct observations cover only a few years out of a vastly longer cycle. This is where additional data came in: more than 25 years of radial velocity measurements of the star — tracking how its motion toward and away from us wobbles under the companion's gravity — plus astrometric data from the Hipparcos and Gaia missions, which capture tiny shifts in the star's position on the sky.
Combining these methods produced a surprisingly precise result. The orbital period of the pair comes out to 1,390 (+310/-200) years. The dynamical mass of the white dwarf is 0.71 (+0.06/-0.05) solar masses. This is particularly valuable because a mass derived directly from orbital dynamics — rather than from theoretical stellar cooling models — allows an independent check on models of stellar evolution.
What this means for the census of our stellar neighbors
HD 38230 is now the 11th-closest known Sirius-like system. That ranking says a lot about how challenging it remains to complete a full census of our nearest stellar neighbors — even within a few dozen light-years of the Sun, undetected companions can still be hiding in plain sight.
The study's authors note that discoveries like this highlight the residual incompleteness of the local white dwarf census. Gaia has given an enormous boost to finding isolated white dwarfs, but those hiding next to bright stars still require dedicated high-contrast imaging surveys. There are likely a few more such hidden companions among our known nearest stars, simply waiting for the right instrument — like Keck — to reveal them.
