Astronomers have established a so-called “Venus zone” to help them single out which exoplanets are most likely to imitate our own inhospitable neighbouring world.
The main goal for NASA’s Kepler mission is to find an exoplanet that is similar to Earth. However, the dip in starlight that betrays the presence of a planet can only reveal the size of the planet; it does not tell us anything about conditions on the planet.
Earth and Venus are similar in size, yet Venus is uninhabitable. Therefore any Earth-sized planet discovered by Kepler does not mean that it is also an Earth-like planet in terms of habitability. So how do astronomers get around this conundrum?
A paper to be published in the Astrophysical Journal Letters by Stephen Kane and colleagues documents the “Venus zone” in an effort to solve this problem. The Venus zone lies alongside the “habitable zone”, and this habitable zone is the region in which an Earth analogue could be found.
The idea of a habitable zone was first proposed in 1993. It is the region around a star where the temperature is just right for liquid water to exist on the surface of a planet. The outer boundary of the habitable zone is marked by the point where the planet succumbs to freezing temperatures and the inner boundary is where a runaway greenhouse effect occurs, and the oceans vaporise.
The Venus zone is a similar idea, and the outer boundary of the Venus zone is also the inner edge of the habitable zone. The inner edge of the Venus zone, also known as the cosmic shoreline, is so close to the parent star that the atmosphere of the planet is removed by the extreme ultraviolet radiation from the star.
Kane tells the writer why it is necessary to have these two different zones. “To understand the history of the Earth, we must also understand the tenuous gap that separates the Earth from a runway greenhouse, such as that which exists on Venus. That understanding will come from determining the frequency of Venus-like planets as well as possible habitable planets like the Earth.”
Kepler is much more suited to finding Venus-like planets than other Earths. This is because Venus-like planets are closer to the star, and thus transit the star more often.
The researchers found 43 planet candidates in the Kepler data that are located in the Venus zone of their stars. The candidates are between half the size of the Earth, and one and a half times the size of the Earth. These candidates have yet to be confirmed as actual planets, but the data is still sufficient to estimate how many Venus-like planets there are. They calculated that around 32 per cent of cool M-dwarf stars should have a Venus-like planet, while 45 per cent of stars more similar to our Sun will come equipped with a Venus.
Answering the question of whether these planets are actually undergoing a runaway greenhouse must be left for future observations.
“The Kepler host stars are quite faint so we need nearby bright stars with Venus candidates for follow-up studies,” says Kane. “These will hopefully be provided by the Transiting Exoplanet Survey Satellite (TESS) and will be able to be monitored in detail using the James Web Space Telescope (JWST). Thus my hope is that in the next 10 years we will have the first confirmations of a Venus-type atmosphere on an exoplanet.”
The paper has been accepted in Astrophysical Journal Letters and the preprint is available here.