News in Science

Hostile Venus Zone given boundaries

Venus Zone Scientists have worked out the borders of the hostile Venus Zone, the area around a star where planets are likely to experience conditions similar to those on the planet Venus.

The Venus Zone is the planetary opposite of the Goldilocks Zone - the habitable zone where an orbiting planet receives enough heat to allow liquid water to exist on its surface.

The research, published in Astrophysical Journal Letters, and on the pre-press website ArXiv.org, will help astronomers determine which Earth-like planets discovered with NASA's Kepler space telescope are actually more analogous to Earth's sister planet Venus.

"We already know where the habitable zone is, what we wanted to find out is the tipping point where an Earth-like planet becomes more Venus-like," says Australian scientist Assistant Professor Stephen Kane of the San Francisco State University.

The scientists have given the Venus zone an inner and outer edge.

The outer edge is where the habitable zone starts - that's the threshold where an atmosphere like Earth's will be pushed into becoming a runaway greenhouse, says Kane.

The inner edge is where a planet loses its atmosphere.

"We calculated that the inner edge is where a planet would lose its atmosphere, because a star's radiation pressure would literally blow a planet's atmosphere away."

Twin planets

Venus and Earth are often described as sister planets. They're about the same size - Earth is just 5 per cent more massive - and both planets are believed to have been made in the same part of the solar system, under similar conditions, and out of the same material.

Earth has oceans of water and relatively moderate temperatures capable of supporting life.

However, if Venus is Earth's sister, it's a twisted sister, with a runaway greenhouse effect where heat trapping carbon dioxide levels have produced surface temperatures of 462°C, hot enough to melt lead.

Venus also has 92 times Earth's atmospheric pressure on its surface. There's no sea level because all the water boiled away long ago, and the only precipitation is sulphuric acid rain, and metallic snow on some mountain peaks.

"This means if you're standing on the surface of Venus you will be melted, crushed and dissolved simultaneously," says Kane.

"These two planets probably had similar atmospheres and similar amounts of water [when they formed] but then they diverged into two completely different worlds. Something changed at one point, and the obvious difference between the two is proximity to the Sun."

Venus is 25 per cent closer to the Sun, and receives almost twice the amount of flux from the Sun.

"We know that's enough to cause a run-away greenhouse, but are there other factors involved as well," asks Kane.

Kane and colleagues found that Earth isn't safely in the middle of the Sun's habitable zone, but very close to the Venus zone boundary.

"In fact it's right up against the edge, if it were any closer to the Sun, Earth's atmosphere would be pushed into a run-away greenhouse, which is what's happening on Venus," says Kane.

Venus-like planets more common

Knowing how common Venus-like planets are elsewhere will help astronomers understand why Earth's atmosphere evolved in ways vastly different from its neighbour.

Previous studies had determined that about 22 per cent of Sun-like stars have Earth-like planets orbiting in their habitable zones.

Kane and colleagues searched the Kepler database for planets between 50 per cent and 140 per cent the mass of Earth, located in the Venus zone of their host stars.

The authors found 43 Earth sized Kepler planets which indicates that about 45 per cent of Sun like stars are likely to host a Venus like planet, twice the number of Earth-like planets.