Widening the "Habitable Zone"

Widening the "Habitable Zone"

(Originally posted February 28, 2017 on Blogger)

A new paper has been published in The Astrophysical Journal Letters which explains how volcanic outgassing on tectonically-active terrestrial exoplanets could allow them to have temperatures warm enough to support liquid water on their surfaces, even if their orbits exist beyond the traditional "habitable zone".

The habitable zone is defined as the circumstellar range of orbits around a star within which rocky planets can support liquid surface water given sufficient atmospheric temperatures and pressures. The details of this definition are too few to have universality in my opinion.

At any rate, we'll recall from this blog, that the 'habitable zone' does not necessarily mean any rocky planet whose orbit is within it will have liquid water pooled on its surface. Nor does it guarantee the planet has any water (regardless of phase) at all. Even if the planet were to have water, there is no guarantee this equates to habitability. In fact, the habitable zone has very little to do with whether a planet is habitable or not. It's merely a guideline; a best first choice to look for terrestrial planets that might have the best chance to harbor life.

There are surfaces within our own solar system that exist outside the habitable zone, yet may harbor life. Some of these surfaces may not be solid at all, such as the upper atmosphere of Venus which has all the ingredients acidophiles here on Earth thrive in. Some moons in our solar system may harbor life without having liquid water pooled on their surfaces, instead having deep subsurface liquid water such as what is theorized beneath the surface of Jovian moons such as Europa, Ganymede, & Callisto, or Saturn's own Titan, and Enceladus.

None of these places exist within our habitable zone (Venus only slightly, and only during a portion of its orbit). In fact, a 2011 paper in the journal Astrobiology describes in great detail how desert worlds with limited surface water can have wider habitable zones than water planets like Earth.

The point I'm trying to make is that a system's 'habitable zone' is merely a guideline at best, and a can be a very rough one at that. In fact, the habitable zone of our own solar system is not perfectly defined; with inner edge estimates anywhere from 0.5 AU (Zsom et al. 2013) to 0.99 AU (Kopparapu et al. 2013) from the Sun, and outer edge estimates ranging from 1.01 AU (Hart et al. 1979) to 2.0 AU (Spiegel et al. 2010).

The paper by Ramses Ramirez and Lisa Kaltenegger adds yet another dimension to all of this by explaining how strong volcanic outgassing could extend the outer edge of our own solar system's habitable zone to ~2.4 AU from the Sun by adding hydrogen (H2).

This, of course, could translate to any planetary system out there, thus extending the circumstellar habitable zone considerably. In an interview with journalists from Science Daily, Ramirez was quoted as saying, ""We just increased the width of the habitable zone by about half, adding a lot more planets to our 'search here' target list..."

Hydrogen (H2) is not directly a greenhouse gas, and since it didn't seem the paper explained this, I'll go ahead and take a stab at it. Hydrogen is the lightest gas, and as such will be displaced by denser air molecules such that it ascends into the upper atmosphere unimpeded.

Along the way, it chemically reacts with hydroxyl radicals (OH), represented by the chemical equation:

OH + H2 ---> H2O (g) + H

where a hydroxyl radical reacts with diatomic hydrogen producing water vapor and atomic hydrogen.

Water vapor is a greenhouse gas. However, more important than the production of water vapor is the scavenging of hydroxyl radicals. The reason this is more important is because hydroxyl radicals are a major sink for methane (CH4), itself a very powerful greenhouse gas.

Hydroxyl radicals act as a methane sink in the following way:

CH4 + OH ---> CH3 + H2O

where methane reacts with a hydroxyl radical producing a methyl group and water vapor. Again, though water vapor is itself a greenhouse gas, it is far less potent than methane.

So imagine, if additional hydrogen exists, then it can greatly reduce the partial pressure of hydroxyl radicals in the atmosphere, thereby reducing the strength of the methane sink, thereby allowing methane to remain in the atmosphere longer than it would have otherwise, thereby enhancing the greenhouse effect of the planet's atmosphere.

What the paper does say about hydrogen as a greenhouse gas, is that "...the potency of hydrogen comes from collision-induced absorption caused by self-broadening from H2-H2 collisions...". Broadening can result in increased greenhouse warming in spectral bands where carbon dioxide and water vapor absorb poorly. If this sounds unfamiliar, I highly recommend you check out my blog on greenhouse gases here: :)

"The Scare Quote Effect & Excited GHGs"

Another thing hydrogen does as a very light gas, is it effectively 'extends' the outer limits of the planet's atmosphere. Making the planet's atmosphere 'bigger' in the sense that it extends outward from the planet's surface further. This would make the planet's atmosphere easier to detect using spectroscopy, and enhance scientists' collective ability to look for exoplanet biosignature gases.

Biosignature gases are gases produced by life, and accumulate to detectable levels in an atmosphere. That detectability could be greatly enhanced by the ability of hydrogen to extend a planet's spectroscopic target. Earth's biosignature gases include nitrogen (N2), carbon dioxide (CO2), and oxygen (O2).

Given all this, we can see how additional hydrogen in an exoplanet's atmosphere could extend the planet's circumstellar habitable zone and increase its detectability. I have to go on a quick tangent here to say that we can't always get something for nothing. Even clean-burning vehicles whose primary exhaust is harmless hydrogen.

Hydrogen reacts with hydroxyls thereby reducing the methane sink as we discussed, but hydrogen also can pass through the tropopause; something the greenhouse gas, water vapor, cannot do. Temperatures at the tropopause are extremely cold, so any water vapor reaching it from the troposphere will freeze out. As such, the tropopause acts as an effective barrier between the lower troposphere (where we live) and the stratosphere, keeping water vapor out of the stratosphere for the most part. 

Layers of Earth's atmosphere, showing the cold tropopause barrier between the very dry stratosphere, and variably humid troposphere.

Layers of Earth's atmosphere, showing the cold tropopause barrier between the very dry stratosphere, and variably humid troposphere.

The stratosphere is therefore very dry by nature. Imagine a bunch of hydrogen getting up there, reacting with hydroxyl radicals, and producing water vapor (a greenhouse gas). Even a 1% increase in water vapor in this portion of our atmosphere could result in an increase of carbon dioxide climate forcing by 40%.

Anyway, detecting collision-induced absorption can be done because of the broader absorption lines it creates, unique from the underlying hydrogen molecules alone. Spectral lines generated from collisions can be five orders of magnitude broader than otherwise 'ordinary' spectral lines to the point that they overlap and appear more as a continua rather than individual lines. This should be recognizable in spectroscopy.

Now we must ask, 'where would an exoplanet get extra hydrogen'?

Ramirez' paper references another paper (Pierrehumbert & Gaidos 2011), which describes how young, unbound planets that accrete massive amounts of primordial hydrogen (up to 40 bars)* can theoretically support liquid water 10 AU from a star like our Sun (G-type). *For reference, atmospheric pressure at sea level on Earth is about 1 bar on average.

The problem with accreted primordial hydrogen is that it doesn't stand the test of geologic time in that it becomes lost to space. Blame this on its 'lightness' relative to other gases. Even at 100 bars, a Super Earth would only be able to hold its hydrogen for ~100 million years.

One likely way for a planet to sustain its ratio of hydrogen, according to Ramirez, is for there to be a continuous source for it such as volcanic outgassing.

This reminds me of when I first learned of the Siberian Traps; the region here on Earth across which a million-year-long volcanic event took place beginning 251 million years ago.

~250 million years ago, Earth looked a lot different than it does today. The orange region highlights the region of the Siberian Traps.

~250 million years ago, Earth looked a lot different than it does today. The orange region highlights the region of the Siberian Traps.

According to the paper, the outgassing of hydrogen and direct greenhouse gases such as methane from that single massive event, resulted in severe global warming that wiped out 90% of life on Earth (The Great Dying), and raised equatorial sea surface temperatures to 40 degrees Celsius (104 degrees Fahrenheit). I'm no expert, but I was unaware volcanic outgassing included a strong hydrogen component... hmmm.. but what do I know?

Anyway, this is an extreme case where outgassing led to a runaway greenhouse effect. Though hydrogen on its own (H2) was a tiny fraction of the overall gasses released, so I'm not entirely sure what kind of role it played (even indirectly) in the runaway greenhouse effect.

However, perhaps somewhere out there a terrestrial planet exists beyond the traditional CH4-CO2-H2O habitable zone, whose outgassing rate of various GHGs is just right to support liquid surface water within the new CH4-CO2-H2O-H2 habitable zone without a runaway GH effect.

Ramirez and Kaltenegger's paper could open the door to new possibilities with regard to finding planets that at least have a chance at supporting life in that excess hydrogen can make a bigger atmospheric target for scientists to look at. Time will tell.

When the James Webb telescope goes into orbit next year, and the European Extremely Large Telescope going into operation in 2024, hydrogen excesses or not, we'll likely be treated to a host of new discoveries across our neighborhood of the Milky Way as these will be better at detecting biosignature gases such as methane in combination with ozone.

Artist's depiction of the James Webb Telescope.

Artist's depiction of the James Webb Telescope.

As always, thanks for reading.

Evidence for Hadean Eon Life?

Evidence for Hadean Eon Life?

The Basics of Inferring Exoplanet Properties

The Basics of Inferring Exoplanet Properties