Hot Jupiters orbit their stars in days. Cold Jupiters orbit in years or decades. The question of whether they coexist in the same system — whether a planet that migrated inward left a sibling behind — bears on how planetary systems form and evolve.
Bonomo et al. (arXiv:2603.12568) examined 28 well-studied Hot Jupiter systems with extensive radial velocity data. Their finding: “the existing RV data are remarkably insensitive to these Hot Jupiter/Cold Jupiter pairs.” The observations that confirmed the Hot Jupiter are nearly blind to a Cold Jupiter in the same system.
The asymmetry is temporal. A Hot Jupiter completes an orbit in days, producing a clear periodic signal in radial velocity data taken over weeks or months. A Cold Jupiter at 3+ AU takes years to orbit. Its signal is a slow drift that looks like a trend, a systematic error, or instrument aging. To detect both siblings, you need observations spanning the Cold Jupiter's orbital period — typically 10-20 years.
Most Hot Jupiter surveys ran for 2-5 years. Long enough to precisely characterize the hot planet. Far too short to see the cold one.
The team found that observation duration dominates detection sensitivity — more important than measurement precision, more important than observation cadence. A modest telescope running for a decade beats a precise telescope running for three years. The bottleneck isn't instrument quality; it's patience.
The implications ripple backward. Current estimates of Hot Jupiter/Cold Jupiter co-occurrence rates are lower bounds at best. The systems where we've confidently declared “no Cold Jupiter companion” may simply be systems where we didn't look long enough. The absence of evidence is not evidence of absence — it's evidence of insufficient temporal baseline. The sibling might be there. We just haven't completed enough of its orbit to notice.