In the Zone. The Venus Zone: Seeking the Twin of our Twin Among the Stars

Artist's concept of lightning on Venus. Image credit: ESA

Artist’s concept of lightning on Venus. Image credit: ESA

What if, in our quest to find another Earth, we happen upon another Venus?

We should celebrate, of course. Venus is often called Earth’s “twin” because it shares of lot of our home planet’s physical characteristics: surface area, composition and density. Also, roughly speaking, both planets inhabit the area around the Sun’s habitable zone – though Venus is near the inner edge, while we on Earth occupy the relative center. Bearing the similarities and differences in mind, scientists Ravi Kumar Kopparapu, Stephen Kane and Shawn Domagal-Goldman explored how distant analogs to Venus might be detected and differentiated from Earth-like planets occupying the same relative space. The paper pinpointing their finding was published in Astrophysical Journal Letters on 9/10/2014.

Successfully detecting analogs of our inner planets out in the Universe, as Kopparapu and colleagues describe, means that at least two important events have taken place.

One says something about us. At present, our ability to identify the existence of other planetary systems is increasing by the day. When it comes to divining which exoplanet is a mini-Neptune and which is a mega-Earth, we still have a ways to go. Gaining the ability to pick out Venus-like planets will imply that we have gotten really good at sorting exoplanets.

The other is a larger statement about the Universe: If Venus-like planets are found in abundance, then Solar Systems like ours may be the rule rather than the exception. Discovering a twin to Venus around another star might well spark our interest in focusing our observations there, both for signs of another Earth and for clues about the dynamics of exoplanetary systems that harbor conditions similar to our own.

In their paper, Kopparapu, Domagal-Goldman and Kane explain how we can determine the distance between a planet and a star from calculations we can make today. They project that in the near future, when the James Webb Space Telescope takes to the skies, measurements of exoplanetary atmospheres will distinguish Venus-like from Earth-like from Mars-like. In the meantime, Kane and colleagues made some important calculations that will assist astronomers in the search for distant Venuses.

Not every planet in or near a habitable zone is habitable. Inhospitable Venus is an excellent example. Credit: NASA/JPL/Caltech

Not every planet in or near a habitable zone is habitable. Inhospitable Venus is an excellent example. Credit: NASA/JPL/Caltech

First, they estimated how close a planet can be to a star and still retain its atmosphere. Those figures pertain to planets like Mercury, with close-in orbits where the Solar Wind strips away nearly all atmospheric particles. Then, they approximated the furthest distance from a star likely to sustain a planet-wide runaway greenhouse effect. Taken together, these parameters describe the Venus zone: a place where we can start looking for planets with characteristics of own second planet from the Sun.

Before we move on to finding planets in other solar systems, we should talk a bit about our own Solar System. Here to do that and then discuss his findings is Dr. Shawn Domagal-Goldman, one of the paper’s authors and the mind behind The Pale Blue Blog at astrobio.net.

Astrobiology Magazine (AM): Shawn, the discovery of just how much Earth and Venus differ is relatively recent. What did it take for us to figure out that our own twin in the Solar System was not just uninhabited, but utterly uninhabitable?

Domagal-Goldman: This is one of the things we’ve learned through telescope and spacecraft observations of Venus over the last century or so. And this highlights two of the things I love about this paper – it leverages what we’ve learned about planets from observations of the ones in our own solar system to inform exoplanet data; and it also reinforces the notion that two planets with fairly Earth-like “astrophysical” properties such as mass/radius can be dramatically different in terms of their habitability.

AM: Where is our own “Venus zone”?

Domagal-Goldman: The outer edge of the “Venus zone” is, by the way we’ve defined it here, the same as the inner edge of the habitable zone. This is roughly the “border” between where we think a planet is more likely to be Venus (closer to the Sun than the border) or more like Earth (further from the Sun than the border). This border therefore is between Earth and Venus. The inner edge of the “Venus zone” is the distance at which Venus would lose it’s atmosphere from all high energy input from the Sun. In our system, this is VERY close to the Sun – about twice as close to the Sun as Mercury is.

AM: We’re just beginning to find rocky planets in so-called habitable zones around other stars. Why is now a good time to start breaking up these habitable zones into discrete bands that reflect Earth and Venus? How will we be able to tell a Venus-zone from an Earth-zone at such tremendous distances?  Will there be a Mars-zone as well, when all is said and done?

In the present-day Solar System with the Sun in its current state, Venus is outside of the habitable zone (HZ) – or rather, it is closer to the Sun than the inner boundary of where water will be found in liquid form. The stability of water as a liquid sets the lines for the traditional habitable zone, outlined here in blue. However, for exoplantary systems with characteristics distinct from ours – where the stars exhibit different luminosity, the planets rotate differently, or the atmosphere conditions vary on those planets vary – the HZ will vary in size and distance from the star. Image Credit: NASA

In the present-day Solar System with the Sun in its current state, Venus is outside of the habitable zone (HZ) – or rather, it is closer to the Sun than the inner boundary of where water will be found in liquid form. The stability of water as a liquid sets the lines for the traditional habitable zone, outlined here in blue. However, for exoplantary systems with characteristics distinct from ours – where the stars exhibit different luminosity, the planets rotate differently, or the atmosphere conditions vary on those planets vary – the HZ will vary in size and distance from the star. Image Credit: NASA

Domagal-Goldman:: There could be a Mars-zone, as well! But getting at that will require us to understand the degree to which Mars was habitable, for how long, and what caused the demise of the red planet’s habitability. These are all questions currently being explored by Curiosity, and we look forward to answers on all those topics.

Ultimately, these sorts of categorizations are going to be done better when we can analyze exoplanets in more detail with bigger future telescopes. The reason we’re doing all this now is for two reasons. First, this gives the community scientific hypotheses for us to test with that sort of mission. Second, it helps us design those missions, and prioritize which objects we would look at first when those missions happen.

AM: The size and location of the Venus zone in each system is going to be dependent on a lot of factors: for example, the luminosity and size of the primary star. A white dwarf star will have a Venus zone much smaller and closer in than our Sun’s. What else will we need to consider in trying to size up Venus zones?

Domagal-Goldman: The other thing that’s really needed now is more simulations of Venus-like atmospheres. This is something that’s very difficult to do, as Venus has been one of the planets that is most difficult to simulate in our computer models. Making advances in that will help us determine the boundaries of both the Venus zone and the habitable zone. Ultimately, we want to define these boundaries with observations from telescopes, but until that happens the best thing we can do will be to improve our simulations and use those results to refine the concept of the Venus zone.

AM: The Kepler Space Telescope has been the workhorse of our planet-hunting mission thus far. When the James Webb takes to the skies, what will change, in terms of finding the Venus Zones, the Venus-analogs, the Earth-analogs and places where we should focus the search for life?

Domagal-Goldman: If we’re lucky, we’ll get a couple Venus-like candidates to study, as the first tests of the hypotheses in this paper. And if we’re extremely lucky, we may get a potentially habitable world or two for us to study, as well. That will be the first mission to move us from studying the “physics” of these planets to being able to study the “chemistry” for a large number of them. Eventually, with a future mission, the goal is to study the biology of such worlds.