Directly Imaging a Young ‘Jupiter’

Centauri Dreams continues to follow the fortunes of the Gemini Planet Imager with great interest, and I thank Horatio Trobinson for a recent note reminding me of the latest news from researchers at the Gemini South installation in Chile. The project organized as the Gemini Planet Imager Exoplanet Survey is a three-year effort designed to do not radial velocity or transit studies but actual imaging of young Jupiters and debris disks around nearby stars. Operating at near-infrared wavelengths, the GPI itself uses adaptive optics, a coronagraph, a calibration interferometer and an integral field spectrograph in its high-contrast imaging work.

Launched in late 2014, the GPIES survey has studied 160 targets out of a projected 600 in a series of observing runs, all the while battling unexpectedly bad weather in Chile. Despite all this, project leader Bruce Macintosh (Stanford University), the man behind the construction of GPI, has been able to announce the discovery of the young ‘Jupiter’ 51 Eridani b, working with researchers from almost forty institutions in North and South America. The discovery was confirmed by follow-up work with the W.M. Keck Observatory on Mauna Kea (Hawaii).

Image: Discovery image of 51 Eri b with the Gemini Planet Imager taken in the near-infrared light on December 18, 2014. The bright central star has been mostly removed by a hardware and software mask to enable the detection of the exoplanet one million times fainter. Credits: J. Rameau (UdeM) and C. Marois (NRC Herzberg).

This is a world with about twice the mass of Jupiter, and this news release from the Gemini Observatory is characterizing it as “the most Solar System-like planet ever directly imaged around another star.” The reasons are obvious: 51 Eridani b orbits at about 13 AU, putting it a bit past Saturn in our own Solar System. And although 51 Eridani b is some 100 light years away, Macintosh and colleagues have found a strong spectroscopic signature of methane.

“Many of the exoplanets astronomers have imaged before have atmospheres that look like very cool stars” says Macintosh. “This one looks like a planet.”

Indeed, and we have further evidence that this is a planet rather than a brown dwarf in chance alignment with the star in the form of a recent paper that analyzes the motion of 51 Eridani b and finds it consistent with a forty-year orbit. Moreover, we’re going to be learning a great deal more about this interesting object in years to come, as the paper explains:

Continued astrometric monitoring of 51 Eri b over the next few years should be sufficient to detect curvature in the orbit, further constraining the semimajor axis and inclination of the orbit, and placing the first constraints on the eccentricity. Absolute astrometric measurements of 51 Eri with GAIA (e.g., Perryman et al. 2014), in conjunction with monitoring of the relative astrometry of 51 Eri b, will enable a direct measurement of the mass of the planet. Combined with the well-constrained age of 51 Eri b, such a determination would provide insight into the evolutionary history of low-mass directly imaged extrasolar planets, and help distinguish between a hot-start or core accretion formation process for this planet.

Image: The Gemini Planet Imager utilizes an integral field spectrograph, an instrument capable of taking images at multiple wavelengths – or colors – of infrared light simultaneously, in order to search for young self-luminous planets around nearby stars. The left side of the animation shows the GPI images of the nearby star 51 Eridani in order of increasing wavelength from 1.5 to 1.8 microns. The images have been processed to suppress the light from 51 Eridani, revealing the exoplanet 51 Eridani b (indicated) which is approximately a million times fainter than the parent star. The bright regions to the left and right of the masked star are artifacts from the image processing algorithm, and can be distinguished from real astrophysical signals based on their brightness and position as a function of wavelength. The spectrum of 51 Eridani b, on the right side of the animation, shows how the brightness of the planet varies as a function of wavelength. If the atmosphere was entirely transmissive, the brightness would be approximately constant as a function of wavelength. This is not the case for 51 Eridani b, the atmosphere of which contains both water (H2O) and methane (CH4). Over the spectral range of this GPI dataset, water absorbs photons between 1.5 and 1.6 microns, and methane absorbs between 1.6 and 1.8 microns. This leads to a strong peak in the brightness of the exoplanet at 1.6 microns, the wavelength at which absorption by both water and methane is weakest. Credit: Robert De Rosa (UC Berkeley), Christian Marois (NRC Herzberg, University of Victoria).

Christian Marois (National Research Council of Canada) discusses the nature of the find:

“GPI is capable of dissecting the light of exoplanets in unprecedented detail so we can now characterize other worlds like never before. The planet is so faint and located so close to its star, that it is also the first directly imaged exoplanet to be fully consistent with Solar System-like planet formation models.”

As you would expect, 51 Eridani b is a young planet, young enough that the heat of its formation gives us a solid infrared signature, allowing its direct detection. In addition to being in an orbit that reminds us of the Solar System, the young world is probably the lowest-mass planet yet imaged, just as its atmospheric methane signature is the strongest yet detected. Given that the Gemini Planet Imager Exoplanet Survey is only a fraction of the way through its observing list, we can expect to find more planets in the target area within 300 light years of the Solar System.

The paper is Macintosh et al., “Discovery and spectroscopy of the young jovian planet 51 Eri b with the Gemini Planet Imager,” Science Vol. 350, No. 6256 (2 October 2015), pp. 64-67 (abstract). The follow-up paper is DeRosa et al., “Astrometric Confirmation and Preliminary Orbital Parameters of the Young Exoplanet 51 Eridani b with the Gemini Planet Imager,” accepted at The Astrophysical Journal Letters (preprint).