A Japanese research team of astronomers and planetary
scientists has used Subaru Telescope's two optical cameras, Suprime-Cam
and the Faint Object Camera and Spectrograph (FOCAS), with a blue
transmission filter to observe planetary transits of super-Earth GJ 1214
b (Gliese 1214 b) (Figure 1).
The team investigated whether this planet has an atmosphere rich in
water or hydrogen. The Subaru observations show that the sky of this
planet does not show a strong Rayleigh scattering feature, which a
cloudless hydrogen-dominated atmosphere would predict. When combined
with the findings of previous observations in other colors, this new
observational result implies that GJ 1214 b is likely to have a
water-rich atmosphere.
Figure 1: Artist's
rendition of a transit of GJ 1214 b in blue light. The blue sphere
represents the host star GJ 1214, and the black ball in front of it on
the right is GJ 1214 b. (Credit: NAOJ)
Super-Earths are emerging as a new type of exoplanet
(i.e., a planet orbiting a star outside of our Solar System) with a mass
and radius larger than the Earth's but less than those of ice giants in
our Solar System, such as Uranus or Neptune. Whether super-Earths are
more like a "large Earth" or a "small Uranus" is unknown, since
scientists have yet to determine their detailed properties. The current
Japanese research team of astronomers and planetary scientists focused
their efforts on investigating the atmospheric features of one
super-Earth, GJ 1214 b, which is located 40 light years from Earth in
the constellation Ophiuchus, northwest of the center of our Milky Way
galaxy. This planet is one of the well-known super-Earths discovered by
Charbonneau et. al. (2009) in the MEarth Project, which focuses on
finding habitable planets around nearby small stars. The current team's
research examined features of light scattering of GJ 1214 b's transit
around its star.
Current theory posits that a planet develops in a
disk of dense gas surrounding a newly formed star (i.e., a
protoplanetary disk). The element hydrogen is a major component of a
protoplanetary disk, and water ice is abundant in an outer region beyond
a so-called "snow line." Findings about where super-Earths have formed
and how they have migrated to their current orbits point to the
prediction that hydrogen or water vapor is a major atmospheric component
of a super-Earth. If scientists can determine the major atmospheric
component of a super-Earth, they can then infer the planet's birthplace
and formation history.
Planetary transits enable scientists to investigate
changes in the wavelength in the brightness of the star (i.e., transit
depth), which indicate the planet's atmospheric composition. Strong
Rayleigh scattering in the optical wavelength is powerful evidence for a
hydrogen-dominated atmosphere. Rayleigh scattering occurs when light
particles scatter in a medium without a change in wavelength. Such
scattering strongly depends on wavelength and enhances short
wavelengths; it causes greater transit depth in the blue rather than in
the red wavelength.
The current team used the two optical cameras
Suprime-Cam and FOCAS on the Subaru Telescope fitted with a blue
transmission filter to search for the Rayleigh scattering feature of GJ
1214 b's atmosphere. This planetary system's very faint host star in
blue light poses a challenge for researchers seeking to determine
whether or not the planet's atmosphere has strong Rayleigh scattering.
The large, powerful light-collecting 8.2 m mirror of the Subaru
Telescope allowed the team to achieve the highest-ever sensitivity in
the bluest region.
The team's observations showed that GJ 1214 b's
atmosphere does not display strong Rayleigh scattering. This finding
implies that the planet has a water-rich or a hydrogen-dominated
atmosphere with extensive clouds. (Figure 2).
Top: If the sky has a clear, upward-extended, hydrogen-dominated atmosphere, Rayleigh scattering disperses a large portion of the blue light from the atmosphere of the host while it scatters less of the red light. As a result, a transit in blue light becomes deeper than the one in red light.
Middle: If the sky has a less extended, water-rich atmosphere, the effect of the Rayleigh scattering is much weaker than in a hydrogen-dominated atmosphere. In this case, transits in all colors have almost the same transit depths.
Bottom: If the sky has extensive clouds, most of the light cannot be transmitted through the atmosphere, even though hydrogen dominates it. As a result, transits in all colors have almost the same transit depths. (Credit: NAOJ)
Although there are only a small number of super-Earths that scientists can observe in the sky now, this situation will dramatically change when the Transiting Exoplanet Survey Satellite (TESS) begins its whole sky survey of small transiting exoplanets in our solar neighborhood. When new targets become available, scientists can study the atmospheres of many super-Earths with the Subaru Telescope and next generation, large telescopes such as the Thirty Meter Telescope (TMT). Such observations will allow scientists to learn even more about the nature of various super-Earths.
References:
- Howe, A.R. & Burrows, A.S. 2012 "Theoretical Transit Spectra for GJ 1214b and other "Super-Earths", Astrophysical Journal, Volume 756, article id. 176.
- Narita, N., Fukui, A., Ikoma, M., Hori, Y., et al. 2013 "Multi-color Transit Photometry of GJ 1214b through BJHKs Bands and a Long-term Monitoring of the Stellar Variability of GJ 1214," Astrophysical Journal, Volume 773, Issue 2, article id. 144.
Acknowledgements:
This research was supported in part by the following:
- Ministry of Education, Culture, Sports, and Technology (MEXT), Japan
- National Institute of Natural Sciences (NINS), Japan
- Japan Society for the Promotion of Science (JSPS), Japan
Source: Subaru Telescope
