Thursday, May 23, 2013

A Potential Super-Venus

Kepler-69c is an exoplanet with ~1.7 times the dimeter of Earth and it orbits around a Sun-like star located about 2700 light-years from Earth. Because it is somewhat larger than Earth, Kepler-69c is in the category of a “super-Earth” type of planetary object. This exoplanet was detected by Kepler - a planet-hunting space telescope. Kepler’s main objective is to detect Earth-size planets within the habitable zone of their parent stars.

In our solar system, the planets Venus and Earth are approximately the same size, with Venus orbiting slightly closer to the Sun than Earth. However, both planets harbour very different surface conditions. The thick carbon dioxide atmosphere on Venus generates a strong greenhouse effect that is responsible for the hellish surface temperatures which makes the planet inhospitable for life. As a result, it is worth considering how likely an Earth-size planet will turn out to be Venus-like or Earth-like, and at what point this divergence is expected to occur.

Figure 1: An artist’s concept of the Kepler-69 planetary system in comparison with the planets of the inner solar system. Kepler-69c orbits near the inner edge of the habitable zone of a Sun-like star. Its 242 day orbit resembles that of Venus in our solar system. The inner Kepler-69b orbits every 13 days and is nowhere near the habitable zone. Credit: NASA/Ames/JPL-Caltech.

Figure 2: Artist’s impression of Venus. The surface of Venus is hidden under a thick and opaque atmosphere.

Kepler-69c orbits its parent star near the hot inner edge of the star’s habitable zone and it receives an estimated flux of 2614 W/m2 from its parent star. In our solar system, the average solar flux Earth receives is 1365 W/m2 while Venus receives 2611 W/m2, or 1.91 times more flux than the Earth. The remarkable similarity in the incident flux received by Kepler-69c and Venus is compelling enough to suggest that Kepler-69c is more likely a super-Venus rather than a super-Earth. This means Kepler-69c probably has a thick carbon dioxide atmosphere with high temperatures and high atmospheric pressure on its surface.

Although the surface of Venus is hot, its upper cloud layers are cool and highly reflective. These clouds cover Venus globally and results in the planet being considerably more reflective than Earth. This means that as a Venus-like planet orbits around its parent star, it is expected to exhibit relatively larger amplitudes of reflected light variations than an Earth-like planet. Observing such a signature can determine whether an Earth-size exoplanet is Venus-like or Earth-like.

Another way to distinguish between a Venus-like or Earth-like planet is to measure the planet’s atmospheric constituents. The atmospheres of Venus and Earth scatter ultraviolet and optical light very differently. Rayleigh scattering is the dominant form of scattering for an Earth-like atmosphere. For Venus, Mie scattering dominates due to scattering of sunlight by droplets of sulphuric acid composing an upper haze layer above the main cloud decks.

Furthermore, the upper region of Venus’ thick carbon dioxide dominated atmosphere is relatively cool due to a strong greenhouse effect within the lower atmosphere which traps infrared radiation at the planet’s surface. As such, a lack of detectable infrared carbon dioxide emissions from the upper atmosphere of a planet may indicate a strong greenhouse effect closer to the surface and suggests that the planet is probably Venus-like. Since clouds of sulphuric acid obscure the lower atmosphere of a Venus-like planet, observations of atmospheric constituents at lower altitudes will be difficult.

By observing the atmosphere of an exoplanet, observatories such as the space-based James Webb Space Telescope (JWST) and the ground-based European Extremely Large Telescope (E-ELT) may be able to distinguish whether an Earth-size exoplanet is Venus-like or Earth-like. The planned Transiting Exoplanet Survey Satellite (TESS) is anticipated to detect more planets like Kepler-69c. Unlike Kepler, TESS will detect exoplanets around stars that are near enough for subsequent follow-up observations with the JWST. The detection of Venus-like planets will help better establish at what point an Earth-size planet is Venus-like or Earth-like.

- Stephen R. Kane et al. (2013), “A Potential Super-Venus in the Kepler-69 System”, arXiv:1305.2933 [astro-ph.EP]
- Ehrenreich et al. (2011), “Transmission spectrum of Venus as a transiting exoplanet”, arXiv:1112.0572 [astro-ph.EP]