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Emergence of the ‘Venus Zone’

In terms of habitability, it’s clear that getting a world too close to its star spells trouble. In the case of Gliese 581c, we had a planet that some thought would allow liquid water at the surface, but subsequent work tells us it’s simply too hot for life as we know it. With the recent dismissal of Gl 581d and g (see Red Dwarf Planets: Weeding Out the False Positives), that leaves no habitable zone worlds that we know about in this otherwise interesting red dwarf system.

I’m glad to see that Stephen Kane (San Francisco State) and his team of researchers are working on the matter of distinguishing an Earth-like world from one that is more like Venus. We’ve made so much of the quest to find something roughly the same size as the Earth that we haven’t always been clear to the general public about what that implies. For Venus is Earth-like in terms of size, but it’s clearly a far cry from Earth in terms of conditions.

Indeed, you would be hard-pressed to find a more hellish place than Venus’ surface. Kane wants to understand where the dividing line is between two planetary outcomes that could not be more different. Says the scientist:

“We believe the Earth and Venus had similar starts in terms of their atmospheric evolution. Something changed at one point, and the obvious difference between the two is proximity to the Sun.”

Kane and company’s paper on this will appear in Astrophysical Journal Letters and is already available on the arXiv server (citation below). At stake here is solar flux, the incoming energy from the planet’s star, which can be used to define an inner and an outer edge to what Kane calls the ‘Venus Zone.’ Venus is 25 percent closer to the Sun than the Earth, but it gets twice the amount of solar flux. Get close enough to the star to trigger runaway greenhouse effects — the results of which make Venus the distinctive nightmare at the surface that it is — and you are at the outer edge of the Venus Zone.

Go further in toward the star and you can pick out the point where a planet’s atmosphere would begin to be eroded by the incoming flux. This is the inner edge of the Venus Zone, and by understanding the boundaries here, we are helping future attempts to characterize Earth-sized worlds in the inner systems of their stars. Find an Earth-sized planet in the Venus Zone and there is reason to suspect that a runaway greenhouse gas effect is in play.


Image: This graphic shows the location of the “Venus Zone,” the area around a star in which a planet is likely to exhibit atmospheric and surface conditions similar to the planet Venus. Credit: Chester Harman, Pennsylvania State University.

The broader picture is an attempt to place our Solar System in context. The Kepler results have consistently demonstrated that any thought of our Solar System being a kind of template for what a system should look like must be abandoned. From the paper:

A critical question that exoplanet searches are attempting to answer is: how common are the various elements that we find within our own Solar System? This includes the determination of Jupiter analogs since the giant planet has undoubtedly played a significant role in the formation and evolution of our Solar System. When considering the terrestrial planets, the attention often turns to atmospheric composition and prospects of habitability. In this context, the size degeneracy of Earth with its sister planet Venus cannot be ignored and the incident flux must be carefully considered.

The study identifies 43 potential Venus analogs from the Kepler data, with occurrence rates similar to those for Earth-class planets, though as the paper notes, with smaller uncertainties. After all, Kepler is more likely to detect shorter-period planets in the Venus Zone than Earth-class planets with longer orbital periods. Overall, the team estimates based on Kepler data that approximately 32% of small low-mass stars have terrestrial planets that are potentially like Venus, while for G-class stars like the Sun, the figure reaches 45%.

Kane notes that future missions will be challenged by the need to distinguish between the Venus and Earth model. We’ll also be looking at the question of carbon in a planet’s atmosphere and its effects on the boundaries of the Venus Zone, the assumption being that more carbon in the atmosphere would push the outer boundary further from the star.

The paper is Kane, Kopparapu and Domagal-Goldman, “On the frequency of potential Venus analogs from Kepler data,” accepted for publication in The Astrophysical Journal Letters and available as a preprint. Be aware as well of the team’s Habitable Zone Gallery, which currently identifies 51 planets as likely being within their star’s habitable zone.


Comments on this entry are closed.

  • Denver September 15, 2014, 12:20

    “We believe the Earth and Venus had similar starts in terms of their atmospheric evolution. Something changed at one point, and the obvious difference between the two is proximity to the Sun.”

    I would think it clear that a process(es) started on Earth that removed all but the tiniest fraction of CO2 from the atmosphere and sequestered it in the crust and mantle. Yet is that enough? Perhaps something like the formation of the Moon removed a significant portion of atmosphere as well?

    Also, I have read that Earth’s blackbody temperature is something like -18c. Does anyone know the Venusian blackbody temperature?

  • Brett September 15, 2014, 12:28

    Some of the papers I’ve seen lately suggest that Venus could have possibly stayed “habitable” if it had much less water (and particularly water vapor), making it impossible for it to have a runaway greenhouse effect. That’s assuming such a “desert world” could be habitable.

  • spaceman September 15, 2014, 12:37

    I noticed from the graphic that Mars is in the outer edge of the habitable zone. Some have argued that the small mass of the red planet has reduced its habitability even though resides in the HZ. If Mars was a larger terrestrial world it would certainly have a better chance of holding a thicker atmosphere. This leads me to the questions: If Mars was the same size as Earth or even a tad larger than Earth… would this make it even more habitable perhaps with a level of habitability somewhere between Earth and actual Mars?

  • Robert September 15, 2014, 12:46

    Has anyone calculated what would happen if we were able to reduce the amount of solar energy Venus gets by enough to make it comparable to earth, how long would it take to cool off and what would likely happen to the toxic chemistry on the planet over time? Just curious. Thanks.

  • Andrew LePage September 15, 2014, 13:05

    I couldn’t help but take these results a touch further and estimate that there are probably about 135 BILLION Venus-analogs in our galaxy (give or take 20-ish billion). Now we need to find the more difficult to detect Earth-size planets in Earth-like orbits in Kepler’s data so that scientists can make meaningful estimates of their occurrence rate.



  • ashley baldwin September 15, 2014, 13:13

    Interestingly all is not necessarily lost in the so called Venus zone. Kopparapu has published previously on the existence of so ” Dune” desert planets within this zone. These are planets that started off with a smaller amount of volatile material, including water. The runaway greenhouse effect can lead to photolysis of water vapour in the planets stratosphere , giving hydrogen, which is lost to space , and oxygen ( which can act as a false positive for life) which is also eventually lost, drying out the planet , and stopping tectonics that lead to the all important carbon cycle by wich carbon is bound with silicates in th crust and kept away from the atmosphere where it can cause greenhouse heating. Water vapour is also an effective greenhouse gas and further aggravates planetary heating if created through carbon dioxide build up in the atmosphere, and especially high up in the stratosphere.. Keep water out of the stratosphere and even a marked greenhouse effect can be managed. Earth has a reasonable amount of water it obtained from the late heavy bombardment of carbonaceous asteroids from the inner belt at 3 AU when Jupiter temporarily migrated inwards before retreating to its current, comet protecting 5.2AU. This wont necessarily be the case in other systems where the inner terrestrial planets may be much drier but potentially resistant to the greenhouse effect nearer to the star. This is coincidentally a big potential impediment to life around M stars where the HBZ is close to the star and further from the “ice line” , that place where it is cold enough for water to exist as ice and where it could come from to hydrate the planet and lubricate the all important climate controlling plate tectonics we encountered above.

  • Galacsi September 15, 2014, 15:35

    Being Nearer to the sun , Venus get 1,991 times more solar irradiance than Earth. Yes but its albedo is much greater (0.9 versus 0.3) because it is covered by whitish clouds.

    So imagine a Venus planet covered with a global ocean or with only Lakshmi Planum ,Ishtar Terra or Aphrodite terra uncovered. Then it just have to be two times move covered in clouds than Earth , to not receive more heat from the sun than our planet. And having much more clouds seems to me obvious because of the highter irradiance.
    How such a planet could ever undergo this runaway greenhouse effect ?

  • Ron S September 15, 2014, 16:35

    Denver: “I have read that Earth’s blackbody temperature is something like -18c. Does anyone know the Venusian blackbody temperature?”

    Seems to depend on where you measure it from. Do you want the surface temperature or what’s observed from afar?

  • ashley baldwin September 15, 2014, 16:46

    Kopparapu also published article earlier this year that showed larger planets with a thicker atmosphere posess less water as a fraction of that, a smaller water column, which keeps it clear of the stratosphere too, allowing them to exist nearer the star without runaway greenhouse. Most of these simulations dont allow clouds either which increase a planets reflectivity, albedo, and help mitigate some of the stellar flux or insolation. They are ALL just simulations though, sophisticated guesses. Totally necessary to bring meaning to observation though. Now we need WFIRST and TESS to have a look to prove or disprove. Penn University tend to the conservative, quite rightly balancing some of the more optimistic models of other Universities. Thats how it works.

  • ashley baldwin September 15, 2014, 16:54

    Dear spaceman, the simple answer is ” yes”. Mars is comfortably inside the HBZ .Even a Venus sized planet might be habitable if it had enough water to produce plate tectonics and vulcanism for a secondary, CO2 dominating and warming atmosphere as well as a magnetic field. Ironically, Mars too might also be habitable if it were swapped with Venus.

  • Eniac September 15, 2014, 17:17

    Galacsi: I think Denver has it correct. The reason Venus is much hotter than Earth despite actually absorbing less light (based on the albedo numbers you mention) is its much thicker CO2 atmosphere. The key factor is not solar proximity, but the presence of life and its role in sequestering carbon. If you take all the carbon dioxide out of Venus’ atmosphere, it becomes a lot more Earth-like, albeit still a bit too dry.

    Also of interest and not yet mentioned here: Because of the faint young sun, 3-4 billion years ago the insolation of Venus was more like that of Earth today, and Earth was much darker than today. Presumably, back then, Earth had a Venus-like CO2 atmosphere that kept it warm enough to be hospitable to early life.

  • andy September 15, 2014, 18:00

    I’ve never been particularly enthusiastic about the “desert planet” extension of the inner habitable zone: it can support the temperature/pressure conditions for liquid water by virtue of not having water, well great but not really all that interesting. Venus itself supports the conditions for liquid water about 60 km above the surface, but there’s not much water there at all (and rather a lot of very concentrated sulphuric acid).

    Of course, that’s before you start considering the large-scale geophysical consequences of having such a water-poor planet. Subduction probably wouldn’t operate so you’d likely be in the stagnant-lid regime, with associated issues with how well various chemical cycles and perhaps a risk of global resurfacing cataclysms…

    Has anyone calculated what would happen if we were able to reduce the amount of solar energy Venus gets by enough to make it comparable to earth, how long would it take to cool off and what would likely happen to the toxic chemistry on the planet over time? Just curious. Thanks.

    Actually, if you factor in that Venus reflects a much larger fraction of the incident sunlight than the Earth does, it turns out that Venus absorbs less energy per unit area than the Earth does. Venus is nothing if not an energy-efficient hellworld.

  • Brett Bellmore September 15, 2014, 18:16

    Notably, a planet like Venus could be a good prospect for colonization. Venus itself has a zone in its atmosphere where both pressure and temperature are comfortable for humans, and breathable air is a good lifting gas in an atmosphere largely composed of CO2.

  • Rob Henry September 15, 2014, 18:35

    It is not as easy to explain how easily Venus lost all its hydrogen as many seem to think. Sure it IS easy to explain how the Cytherean cold trap for water quickly migrated so much higher than its terrestrial equivalent, but its gravity is far to deep to lose any significant quantity by thermal mechanisms, and it stretchers theory for non-thermal mechanisms to do it. I wouldn’t even be surprised if it later turned out that the flares and solar winds of red dwarfs turned out to add more hydrogen than they took.

    Such a ‘Venus’ would still be Hell on the ground, but with a warm and very wet zone high in the atmosphere that might turn out to be an even better environment for life than those we are more familiar with. Best not assume too much in the hunt for life.

  • Abelard Lindsey September 15, 2014, 18:57

    One difference between Earth and Venus is that Earth has plate tectonics and Venus does not. Plate tectonics are believed necessary for maintaining habitability for various reasons. If the giant impact (that made the Moon) is the reason why Earth has plate tectonics, it is reasonable to assume that there are a lot more Venus’s than Earth’s out there, not to mention the corresponding rareness of life to go along with it.

  • Andrew LePage September 15, 2014, 19:14

    @Denver September 15, 2014 at 12:20

    The majority of Earth’s atmospheric CO2 is locked up in carbonate deposits primarily generated by the water-based weathering of silicates on the surface (this is part of the carbonate-silicate cycke). I’ve seen estimates that up to 60 bars worth of CO2 is locked up in Earth’s carbonate deposits (comparable to Venus’ allotment of this greenhouse gas). The difference between us and Venus is that Venus lost all of its water very early in its history due to a runaway moist greenhouse effect that allowed water vapor high into its atmosphere where solar UV could break it down into hydrogen and oxygen (with the escape of the hydrogen leading to a dessication of Venus). Without liquid water to dissolve the atmospheric CO2 to start the carbonate-silicate cycle, Venus’ allotment of CO2 remains in its atmosphere to fuel the planet’s current dry runaway greenhouse.

    And to answer your other question, my quick “back of the envelope” calculation for the blackbody temperature of Venus is about 190 C or 375 F.

  • Phil September 15, 2014, 21:38

    Slight tangent, but I dont think we’ve heard the last from GJ 581 and the other stars (GJ 667 etc) that have recently ‘lost’ super-earths by a long shot (who knows – may we even hear again from Vogt et al regarding these matters?).
    Apart from anything alse – clearing away these false positives from the current noise floors should hopefully eventually allow a range of Earths (0.5>1.5 Me) to emerge at sub Msec reflexes once a new generation of ultra stable instruments (laser combs / 30m class telescopes) come on line. Certainly these systems can dynamically fit these putative planets. Its going to take time, but maybe in ten years we will be looking back on the last ten years as a bit of a premature dawn for radial velocity exoplanets in the Hz?

  • ashley baldwin September 16, 2014, 3:31

    Plate tectonics is due to a planet having a hot, ” plastic” mantle that facilitates convection of heat up from the even hotter plsnetry core. The crust sits on this as plates which are moved by this convection. The heat comes from a mixture of the residue of formation and radioactive mineral decay and escapes from the seam st the centre of oceanic plates which are made of basalt and are therefore denser than the continental plates. The new material welling up from the mid oceanic fault lines push the plates outwards until they meet the lighter , granite , continental plates where they slide underneath at ” subduction zones “and back down into the cooler surface mantle as it cools and sinks back down to the hotter core , starting the process off again. The friction as they do this creates heat at the subduction zones that causes vulcanism. Some of the carbon tied up by the carbon cycle which washes CO2 out of the atmosphere and into the oceans where it sits benignly as carbonate rock , along with seawater , is tied up in this and released back into the atmoslhere as CO2 gas and water vapour, to start the carbon cycle again too. All of this is driven by the Earths internal heat source which is dependent on its size to a degree ( due to it having more radioactive isotopes like uranium and potassium that produce heat as they decay) and the residue of its formation and the whole plate tectonics process is lubricated by water to an extent. The formation of the moon is believed to be due to a glancing strike from a Mars sized body about 3.8 billion years ago. Apart from adding a bit of heat it had little do do with tectonics. Mars has no tectonics because it is too small and has lost jts internal heat, and Venus-like planets dont have it because runaway greenhouse has lost all of their water , taking away its critical lubricating role . The process is descibed in beautiful detail in the great pop science book about M dwarfs, ” Under a crimson sun”.

  • norman wells September 16, 2014, 4:46

    It’s not possible is it, that Venus is the awful warning , of the ultimate consequences of runaway global warming? Meaning that ,perhaps it was originally an inhabited planet at a more advanced stage of Evolution than Earth .which was overcome by global warming

  • Michael Spencer September 16, 2014, 6:57

    Kim Stanley Robinson has explored ideas for cooling Venus with some sort of solar shade in several recent novels – where, parenthetically, he posits life on Mercury that depends on a sort of super-railway encircling the planet, always ahead of sunrise.

  • ljk September 16, 2014, 10:12
  • ashley baldwin September 16, 2014, 11:48

    I think the evidence from those few landers to reach the surface of Venus point to it having a more passive climate in the past , but not for long and although it had water to begin with this was likely to be in steam form , rapidly being broken down and lost, leaving an irreversible smothering blanket of a thick CO2 atmosphere that more than compensates for a high albedo sadly.

  • ashley baldwin September 16, 2014, 12:46

    Good for you ljk. Thats why we need to get up there and start looking rather than running interminable computer simulations. NASA ( and others) has the technology to do this, and some, its just a question of money and will now. $1.6 billion and the NRO mirror does it nicely. Just heard that tbe formation flying is no harder than docking at the ISS, so good to go. Would have been less if they had let the JWST be wired up for a starshade . Now that would have been an exoplanet telescope!

  • Eniac September 16, 2014, 20:16

    The majority of Earth’s atmospheric CO2 is locked up in carbonate deposits primarily generated by the water-based weathering of silicates on the surface (this is part of the carbonate-silicate cycke).

    I think the inorganic production of carbonates that you describe is dwarfed by biological fixation, evidenced by the fact that most limestone originates from marine organisms.

  • ljk September 17, 2014, 10:08

    September 16, 2014

    Early Earth’s Hadean Era –“Surprising Similar to Today –With Oceans, Continents and Tectonic Plates”

    Conditions on Earth for the first 500 million years after it formed may have been surprisingly similar to the present day, complete with oceans, continents and active crustal plates. This alternate view of Earth’s first geologic eon, called the Hadean, has gained substantial new support from the first detailed comparison of zircon crystals that formed more than 4 billion years ago with those formed contemporaneously in Iceland, which has been proposed as a possible geological analog for early Earth.

    From the early 20th century up through the 1980’s, geologists generally agreed that conditions during the Hadean period were utterly hostile to life. Inability to find rock formations from the period led them to conclude that early Earth was hellishly hot, either entirely molten or subject to such intense asteroid bombardment that any rocks that formed were rapidly remelted. As a result, they pictured the surface of the Earth as covered by a giant “magma ocean.”

    This perception began to change about 30 years ago when geologists discovered zircon crystals (a mineral typically associated with granite) with ages exceeding 4 billion years old preserved in younger sandstones. These ancient zircons opened the door for exploration of the Earth’s earliest crust. In addition to the radiometric dating techniques that revealed the ages of these ancient zircons, geologists used other analytical techniques to extract information about the environment in which the crystals formed, including the temperature and whether water was present.

    Since then zircon studies have revealed that the Hadean Earth was not the uniformly hellish place previously imagined, but during some periods possessed an established crust cool enough so that surface water could form – possibly on the scale of oceans.

    Accepting that the early Earth had a solid crust and liquid water (at least at times), scientists have continued to debate the nature of that crust and the processes that were active at that time: How similar was the Hadean Earth to what we see today?

    Full article here:


  • Andrew LePage September 17, 2014, 11:30

    @Eniac September 16, 2014 at 20:16

    While it is certainly true that biological fixation of CO2 is an important process here on Earth today, James Kasting et al. in their early work on the carbonate-silicate cycle demonstrated that abiotic production of carbonates is possible and probably was the dominant means of producing carbonate deposits here on Earth before life started making shells and other carbonate-based structures a half a billion or so years years ago.

  • Rob Henry September 17, 2014, 16:11

    Oops, I was thinking in a far too simplistic mode above. If you give Venus, an Earth like magnetic field or significant free atmospheric oxygen, that would heat up the exobase to temperatures where hydrogen loss is only diffusion limited, but deuterium and helium are lost much more slowly. Free oxygen means ozone above water’s cold trap, so it is still easy to find mechanisms to retain water, but the picture is even more complex than I wrote above.

  • ljk September 19, 2014, 10:26

    September 18, 2014

    Cosmic Puzzle Solved? –“Why Every Organism on Earth is a Right-handed Double Helix”

    The DNA of every organism on Earth is a right-handed double helix, but why that would be has puzzled scientists since not long after Francis Crick and James Watson announced the discovery of DNA’s double-helical structure in 1953. It’s a puzzle because no one has been able to think of a fundamental reason why DNA couldn’t also be left-handed.

    New research by University of Nebraska-Lincoln physicists and published in the Sept. 12 online edition of Physical Review Letters now gives support to a long-posited but never-proven hypothesis that electrons in cosmic rays — which are mostly left-handed — preferentially destroyed left-handed precursors of DNA on the primordial Earth.

    The hypothesis, called the Vester-Ulbricht model, was proposed by Frederic Vester of the University of Saarbrucken in Germany and Tilo L.V. Ulbricht of the University of Cambridge in England in 1961 in response to the 1957 discovery that most of the electrons spewing from radioactive beta decay were left-handed.

    Joan M. Dreiling and Timothy J. Gay of UNL focused circularly polarized laser light on a specially prepared crystal of gallium-arsenide to produce electrons whose spins were either parallel or anti-parallel to their direction of motion upon emission from the crystal — essentially artificial beta rays. They then directed these electrons to strike target molecules of a substance called bromocamphor, which comes in both right- and left-handed varieties.

    Full article here:


  • Eniac September 19, 2014, 16:38


    Cosmic Puzzle Solved? –”Why Every Organism on Earth is a Right-handed Double Helix”

    Nonsense if I ever heard any. Why is this a puzzle? Would you be less puzzled if DNA was left-handed? There are only exactly two possibilities, after all. Neither should puzzle us in the least, particularly if they are exactly equally likely (The sanest hypothesis).

    Or are you puzzled by the fact that both don’t coexist? The answer to this question is to be sought in the dynamics of evolution and has not a thing to do with cosmic rays.

  • ljk September 29, 2014, 9:54

    How Viewing Earth as an Exoplanet Can Help Search for Alien Life

    By Elizabeth Howell, Astrobiology Magazine | September 29, 2014 07:00 am ET

    An extraterrestrial spacecraft lurking in a satellite’s orbit near Earth would be able to see city lights and pollution in our atmosphere. But what if it searched for signs of life on Earth from afar?

    This question has great pertinence to those searching for other Earths outside of our solar system. NASA’s Kepler space telescope is among a fleet of telescopes and spacecraft searching for rocky planets similar to our own. Once the size and location of these worlds are plotted, the next step is examining the chemical composition of their atmospheres.

    From afar, Earth-like worlds appear as tiny points of light, making it hard to imagine ever finding out much about them. The best we can do with telescope technology at the moment is to examine some atmospheric components of worlds that are larger than Jupiter. But that doesn’t mean we should discount the possibility of ever finding a planet similar in size to our own, researchers say. Telescopes are only getting more powerful.

    Full article here:


  • ljk December 30, 2014, 15:52

    Scorching Hot Venus May Have Once Had Oceans – of Carbon Dioxide

    By Paul Scott Anderson

    The continued exploration of the planets and moons in our solar system have revealed many strange things, including that Earth is not the only place with oceans and seas. Mars once had lakes and possibly oceans in the distant past, some of the icy moons in the outer Solar System such as Europa and Enceladus currently have subsurface oceans and seas and Titan has seas and lakes of liquid methane/ethane on its surface. That’s weird enough, but now there’s a new twist: Venus may have had oceans of liquid carbon dioxide (CO2) in the past


  • Ronald February 4, 2015, 9:04

    Late comment for the record:
    While I find the definition of the ‘Venus-zone’ less interesting, it simply being a zone on the inside (i.e. smaller AU distance) of the HZ as defined by Kasting, Kopparapu et al., the really interesting issue here, I think is whether the abundance of small terrestrial planets can be extenden, extrapolated toward greater AU, i.e. into the HZ., as also sugegsted by Andrew LePage (September 15, 2014 at 13:05).
    In stark contrast to the mentioned abundance of the Venus analogs is the rather low frequency of earth analogs as per Foreman-Mackey et al. (2014) of only 1.6% per solar type star. It should be mentioned that they define the orbital zon too narrow (200 – 400 days) for the HZ, but on the other hand the upper limit of an earth analog planet too high (2Re).