How many habitable worlds like our own could exist around other stars? Since the discovery of the first exoplanets, the answer to this question has seemed tantalizingly close. But to estimate the number of Earths, we first need to understand how our planet could have gone catastrophically awry.
In other words, we need to return to Venus.
We have now discovered over 4000 planets beyond our solar system. Approximately one-third of these worlds are Earth-sized and likely to have rocky surfaces not crushed under deep atmospheres. The next step is to discover how many of these support temperate landscapes versus ones unsuitable for life.
The Earth’s habitability is often ascribed to the level of sunlight we receive. We orbit in the so-called ‘habitable zone’ where our planet’s geological cycle can adjust the level of carbon dioxide in our atmosphere to keep our seas liquid. In a closer orbit to the sun, this cycle could not operate fast enough to keep the Earth cool. Our seas would evaporate and our atmosphere fill with carbon dioxide, sending the planet temperature into an upwards spiral known as a runaway greenhouse.
If our solar system had just one Earth-sized planet, this would suggest we could simply count-up similar sized planets in the habitable zones around other stars. This would then be our set of the most likely habitable worlds.
However, this idea is shredded in a new paper posted this month to be published in the Journal of Geophysical Research: Planets. Led by Stephen Kane from the University of California, Riverside, the paper is authored by many of the top planetary scientists we have met before in this column.
Their message is simple: our sun is orbited by two Earth-sized planets but only one is habitable. To identify habitable planets around other stars, we need to explain why the Earth and Venus evolved so differently. And the data suggests this is not just a climate catastrophe.
Orbiting beyond the inner edge of the habitable zone, Venus does appear at first to be a runaway Earth. The planet’s atmosphere is 96.5% carbon dioxide, smothering the surface to escalate temperatures to a staggering 863°F (462°C). Images from NASA’s Pioneer Venus mission in the late 1970s revealed a surface of highlands and lowlands that resembled the continents of Earth. This is all consistent with a picture of an Earth-like planet with a runaway greenhouse atmosphere.
But then the mysteries begin. Water evaporating from the planet surface would be split by the ultraviolet rays from the sun into hydrogen and oxygen. Hydrogen is too light to be held by Venus’s gravity and would disappear into space, leaving the oxygen behind. A post-runaway Venus should then have an oxygen-rich atmosphere, which we do not observe.
Also, the surface of Venus turns out to be very young at around 20% of the age of the planet. Only about 1000 impact craters are visible, implying that the Venusian surface has renewed and smoothed over older craters. The Earth also renews its surface through events such as volcanic activity which spews fresh rock onto our crust. But this Earth geology results in an amalgamation of young and olds sections, whereas nearly the entire surface of Venus seems to be similarly young. This points to a very un-Earth-like geological history and different internal structure.
Then there is the environment of early Venus. The young sun was cooler than today, which may have allowed water and even life to flourish. Alternatively, the planet may always have been too warm for water to condense.
There are multiple theories for explaining the history of Venus, but the data is not available to prove them. This leaves us with no guarantee that the inner edge of the habitable zone marks the boundary between an Earth-like and Venus-like surface. Habitable worlds could flourish for billions of years outside the habitable zone, while the Venus-catastrophe may regularly occur on planets that should be temperate.
But Kane’s paper is not a list of problems. It is a solutions paper. Upcoming instruments such as NASA’s James Webb Space Telescope (JWST) and ESA’s ARIEL mission will be able to detect the composition of a planet’s atmosphere. This potentially can distinguish between an Earth and a Venus.
A couple of particularly interesting candidates are TRAPPIST-1d and K2-3d; two rocky planets that orbit on the brink of the inner edge of the habitable zone. If these two have Venus-like atmospheres, it would support the idea that starlight is the driving factor determining whether a terrestrial world becomes a Venus or an Earth. If they do not, then we need to consider more paths for terrestrial planet evolution.
But there is a caveat. Correctly interpreting observations of exoplanet atmospheres requires accurate models of what occurs between the observable top of the atmosphere to the invisible deeper layers towards the surface. If this is wrong, then a burning planet could be thought temperate or vice versa.
“We will never have in-situ data for the surface of an exoplanet,” points out Stephen Kane. “That means we will only ever infer exoplanet surface conditions from models.”
Evidence for how wrong we could be is immortalised in the 1965 science fiction movie, ‘Voyage to the Prehistoric Planet’, which depicts the surface of Venus as a wet, life-filled world. An Earth-like atmosphere around Venus would give the planet an average surface temperature of about 81°F (27°C), making this a respectable guess. When the Soviet Venera 4 probe descended for the first time towards the Venusian surface in 1967, it was designed to float in case it hit oceans. The probe never hit the surface, returning the message that Venus was extremely hot before falling permanently silent.
Models that link the observable atmosphere to the surface are based on data from the planets we can visit in our own solar system. But our current models still fail to predict the horrendous surface conditions on Venus.
“If we can’t get this right for the nearest Earth-sized planet then I do not currently have confidence that we can reliably infer anything about exoplanet surface conditions,” says Kane. “This is a critical problem in the search for habitable exoplanets.”
To address this issue, we need to collect more data from Venus about the cloud composition, pressures and temperatures through the atmosphere. And while we are there, there are other measurements that could help decipher Venus’s own history.
Measurements of the noble gases such as xenon, argon and neon are particularly interesting as they are extremely unreactive. Changes in their abundance can therefore only occur through a limited number of methods.
For example, research by Kevin Zahnle from NASA Ames (and co-author with Kane) found that the most likely way xenon would be lost from a planet is by being dragged into space by a hydrogen-rich wind. Such a wind would occur as Venus lost its water when the sun split the evaporated molecules apart. Meanwhile, an isotope of xenon with a slightly different mass can be gradually replenished on a planet by the radioactive decay of other atoms. The relative abundance of these two xenon types could be used to date when Venus lost its water; the higher the quantity of replenished xenon, the more time has passed since the hydrogen wind whisked water and xenon away from the planet. A timestamp for this process would indicate how long Venus might have been habitable.
Born around the same star, these comparisons between our two Earth-sized worlds are a unique look at the evolution of a habitable environment. Both worlds should have been formed with the ingredients for life but only one planet maintains the necessary conditions today.
Venus’s story is therefore the key to discovering how common habitable planets might be in the Galaxy. We need to know not just what conditions could form a habitable world, but also what would make it fail.
Elizabeth Tasker is an astrophysicist and science communicator at the Japan Aerospace Exploration Agency (JAXA). Her research explores the formation of stars and planets, while her science articles have covered topics from Egyptian coffins to deep sea drilling (but mainly focus on exoplanets and space missions!). She is the author of “The Planet Factory” on the formation of planets and the strange worlds we have discovered beyond our Sun and also keeps her own website and personal blog.