HIP 78530 b

Lore & Background

HIP 78530 b was first detected as a faint object between 2000 and 2001 using the ADONIS system at the ESO 3.6 m Telescope in Chile. Reports in 2005 and 2007 could not confirm whether it was an orbiting companion, and the team did not follow up. The object was rediscovered on May 24, 2008, during a direct imaging survey of ninety-one stars in the Upper Scorpius association using the Gemini Observatory's NIRI and ALTAIR adaptive optics system.

Follow-up imaging on July 2, 2009, and August 30, 2010, measured its proper motion, confirming it as a bound companion. Spectroscopic data collected on July 2, 2009, July 3, 2009, and August 8, 2009, using the NIFS integral field spectrograph with ALTAIR, helped confirm HIP 78530 b's nature. The discovery was reported on January 24, 2011.

The host star, HIP 78530, is a luminous blue B-type main sequence star in the Upper Scorpius association, a loose star cluster. The star is about 2.2 times the mass of the Sun, with an effective temperature of 10,500 K. The age of the Upper Scorpius group is estimated at 5 million years, though a more recent estimate suggests approximately 11 million years. HIP 78530 has an apparent magnitude of 7.18, making it faint or invisible to the unaided eye.

HIP 78530 b is most likely a brown dwarf, massive enough to fuse deuterium but not to ignite as a star. Its estimated mass is over twenty-three times that of Jupiter, and it orbits at a semimajor axis roughly 710 AU from its star—about eighteen times farther than Pluto's average distance from the Sun. It completes an orbit approximately every 12,000 years on a somewhat eccentric path. Only 0.2% of its orbit has been tracked over a 16-year baseline, so orbital parameters remain uncertain.

Reader's Guide

The discovery of HIP 78530 b, along with 1RXSJ1609-2105b, allowed astronomers to predict that bodies with planet/brown dwarf-to-star mass ratios below 0.01, orbiting at hundreds of AU, exist around 2.2% of all stars. This number is considered a lower limit, as smaller, low-mass planets in such configurations remain undetectable with current technology. HIP 78530 b's mass straddles the conventional boundary between planet and brown dwarf, prompting astronomers to investigate its formation mechanism—whether it formed like a planet or like a star.

The object's wide orbit and young host star make it a valuable case study for understanding the formation and evolution of substellar companions at large separations. Its detection via direct imaging, a technique that captures light from the companion itself rather than inferring its presence through transit or radial velocity, demonstrates the power of adaptive optics in resolving faint objects near bright stars. The long orbital period and incomplete orbital coverage mean that many details of HIP 78530 b's orbit and physical properties remain to be refined with continued observation.