β Pictoris b: An Exoplanet With the Atmosphere of a Brown Dwarf

March 23, 2017

Using the Gemini Planet Imager (GPI), a team of astronomers led by J. Chilcote (University of Toronto) found that the low mass stellar companion β Pictoris b is about 13 times more massive than Jupiter with a surface temperature of about 1720 K. While these parameters are in good agreement with earlier observations, they allow better comparisons with planetary evolution models. The Gemini data indicate that β Pictoris b best matches an exoplanet with an atmosphere like that of a low-surface gravity (L21) brown dwarf.

Observations at the Gemini South telescope in Chile, using the Gemini Planet Imager (GPI), are refining our understanding of the β Pictoris system. The system contains the ~ 13 Jupiter mass companion β Pictoris b which is at the mass boundary sometimes used to distinguish between an exoplanet and a brown dwarf. Brown dwarfs are objects that are not massive enough for sustained nuclear reactions. Brown dwarfs less massive than 13 Jupiters cannot even start a nuclear reaction.

Based on the GPI data, combined with planetary evolution and atmospheric models, Chilcote suggests a “hot-start” planet formation scenario for β Pictoris b. He adds, “This is consistent with the disk instability formation mechanism for wide-orbit giant exoplanets.” However, the characteristics for the atmosphere of β Pictoris b found in this work best match those of low-surface gravity brown dwarfs, not planets.

The team studied β Pictoris b during the verification and commissioning of the Gemini Planet Imager and as part of an astrometric (position) monitoring program designed to constrain the orbit of the exoplanet. This work is also part of a Gemini Large and Long Program.

“With GPI the Gemini Observatory is at the forefront of exoplanet exploration,” said Chilcote. He adds, “Direct imaging allows for the discovery of planets on solar systems-scale orbits, provides new insight into the formation and characteristics of extrasolar systems, and enable direct spectroscopic observations of their atmospheres.”

Since the first detection of an exoplanet in 1995 (51 Pegasi b) the discovery and characterization of extrasolar planets has changed the understanding of planetary systems and their formation. Over the past two decades, more than 3400 of planetary systems with stars of various masses and at different stages of evolution have been detected. Some of these planetary systems present features very similar to our solar system. The current challenge for astronomers is to better characterize these planets, especially the exoplanet atmospheres which can give us information about the history of formation of the planets.

Abstract

Using the Gemini Planet Imager (GPI) located at Gemini South, we measured the near-infrared (1.0– 2.4 µm) spectrum of the planetary companion to the nearby, young star β Pictoris. We compare the spectrum obtained with currently published model grids and with known substellar objects and present the best matching models as well as the best matching observed objects. Comparing the empirical measurement of the bolometric luminosity to evolutionary models, we find a mass of 12.9 ± 0.2M_Jup, an effective temperature of 1724 ± 15 K, a radius of 1.46 ± 0.01 R_Jup, and a surface gravity of log g = 4.18 ± 0.01 [dex] (cgs). The stated uncertainties are statistical errors only, and do not incorporate any uncertainty on the evolutionary models. Using atmospheric models, we find an effective temperature of 1700 − 1800 K and a surface gravity of log g = 3.5–4.0 [dex] depending upon model. These values agree well with other publications and with “hot-start” predictions from planetary evolution models. Further, we find that the spectrum of β Pic b best matches a low-surface gravity L2 ± 1 brown dwarf. Finally comparing the spectrum to field brown dwarfs we find the the spectrum best matches 2MASS J04062677–381210 and 2MASS J03552337+1133437.