Figure 2 . GNIRS spectrum of the 250 K brown dwarf , WISE 0855 .
WISE 0855 is our first opportunity to study an extrasolar planetary-mass object that is nearly as cold as our own gas giant planets .
The difficulty of studying exoplanets is that they are much fainter than their host stars . Dedicated instruments , such as the Gemini Planet Imager , can detect the light of warm Jupiter-mass analogs . However , the capability does not yet exist to image a planet as cold as Jupiter around another star .
An alternative approach is to study freefloating planets and brown dwarfs . These objects slowly cool as they radiate away the energy from their gravitational collapse , with no core fusion to create new energy . Brown dwarfs can be found over a much wider temperature range than exoplanets . And temperature , rather than mass , dominates the appearances of self-luminous planets and brown dwarfs .
By far the best extrasolar analog to Jupiter is the brown dwarf WISE 0855 . Kevin Luhman of Pennsylvannia State University discovered this free-floating object in 2014 while searching Wide-field Infrared Survey Explorer ( WISE ) satellite data for extremely red objects with high proper motions . Using the NASA Spitzer Space Telescope , Luhman determined that WISE 0855 is just two parsecs from the Sun ; together with its photometry , this implies an effective temperature of ~ 250 K ( the coldest known compact object outside of our Solar System ) and a mass of 3-10 M Jupiter
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When WISE 0855 was discovered , a flurry of interest in characterizing its atmosphere ensued . Models predict that at 250 K , WISE 0855 should have a spectrum dominated by water vapor , phosphine , and perhaps a subtle influence from water clouds . But the method typically used to study brown dwarf atmospheres — nearinfrared spectroscopy ( 1-2 microns ) — is infeasible on current facilities due to WISE 0855 ’ s intrinsic faintness ( J = ~ 25 magnitudes ). Counterintuitively , the best way to obtain a spectrum of WISE 0855 is with ground-based M-band ( 5 micron ) spectroscopy , which , due to the sky background brightness , is usually far less sensitive than other wavelengths . As WISE 0855 has an M-band magnitude ( measured from WISE ) of 13.9 , it is easier to detect at M-band than J-band . There are currently no space-based 5-micron spectrographs .
Enter Gemini
The previous faintest spectrum ever taken from the ground at M-band was a Gemini Near-Infrared Spectrograph ( GNIRS ) spectrum of Gliese 570 D which is 1.6 magnitudes brighter than WISE 0855 . Scaling from previous observations , a low-resolution , low signal-to-noise GNIRS spectrum of WISE 0855 was just barely possible in a 14-hour integration ( 29 hours including overheads ). But there are always practical considerations when working at an instrument ’ s limits . Could we keep an invisible object moving 8 arcseconds per year in the slit for 29 hours over the course of many nights ? Would we see enough of a trace in two-hours clock time
4 GeminiFocus October 2016