Exoplanets

A New Place to Search for Habitable Planets: “The Soot Line.”

The habitable zone is the region around a star where planets can maintain liquid water on their surface. It’s axiomatic that planets with liquid water are the best places to look for life, and astronomers focus their search on that zone. As far as we can tell, no water equals no life.

But new research suggests another delineation in solar systems that could influence habitability: The Soot Line.

Even though Earth is 2/3 covered in oceans, it’s still considered a water-poor planet. Our planet is only about 0.1% water by mass. But, obviously, that water is critical for life. Cells can’t perform their functions without water.

But carbon is critical for life, too, and Earth life is called carbon-based life. Carbon is unique in its ability to form diverse compounds, and to form polymers at Earth temperatures. It’s the second-most abundant element in the human body, after oxygen.

This illustration helps show how critical carbon is to all life. Here on Earth, cell membranes are composed of phospholipid molecules dissolved in liquid water. A phospholipid has a backbone of carbon atoms (gray) and also contains hydrogen (sky blue), phosphorus (yellow), oxygen (red), and nitrogen (blue). Credit: Ties van Brussel

But Earth is actually carbon-poor, and only 0.024% of the planet’s crust is carbon. That’s because it formed inside the Solar System’s soot line, where carbon is less plentiful. But what if sooty exoplanets that form outside a solar system’s soot line contain significantly more carbon? How could that affect habitability?

This is an artist’s impression of the super-Earth 55 Cancri e in front of its parent star. The exoplanet has far more carbon than Earth does, though it’s much too hot for life. Credit: ESA/NASA

That’s what a new paper urges us to consider.

The new paper is titled “Exoplanet Volatile Carbon Content as a Natural Pathway for Haze Formation,” and it was published in The Astrophysical Journal Letters. The lead author is Edwin Bergin, Professor and Chair of Astronomy at the University of Michigan.

What is the Soot Line?

The search for habitable exoplanets focuses on water and also on planets that formed similarly to ours. Planets form in protoplanetary disks that rotate around young stars, and those stars have a huge effect on the nature of the materials in the disk, including carbon. The Sun’s heat defines different frost lines at different distances from the Sun, and each frost line is relevant to a different volatile. Outside a particular frost line, pressure and temperature force a particular volatile to remain solid, while inside that line, the same volatile can sublimate to vapour.

Since solids are more likely to stick around and form planets, the region inside a particular chemical’s frost line will have fewer solids of that chemical, making it less available for planetary formation. This is why, or partly why, there’s so little water on Earth, while frozen water is abundant further away from the Sun. Earth formed inside the water frost line, and much of its water was most likely delivered later.

This image shows the relative locations of a solar system’s frost line and soot line. Image Credit: NASA

The soot line is similar to a frost line, but it concerns life-critical carbon. It’s the distance from a star where there’s not enough heat to vaporize carbon, and instead, the carbon in molecules stays solid. Since it’s solid, it’s more readily available for planet formation, so planets that form outside of the soot line should be more carbon-rich than Earth.

If Earth formed inside the soot line, then heat from the young Sun would’ve turned carbon-rich compounds into gas, which disperses much more easily. That meant there was less carbon in the solids that formed Earth, so Earth ended up with less carbon. Makes sense from our perspective.

When it comes to planets, habitability, and frost lines, the lines for water and carbon monoxide (CO) are of prime importance for exoplanet scientists. That’s because scientists think that these two compounds are the prime carriers of oxygen and carbon, both critical for life. The CO line is further away from the Sun by tens of AU than the water line because CO freezes at a much lower temperature. This should mean that planets closer to their stars, like Earth, have less carbon.

Most carbon is contained within CO, methane (CH4,) and carbon dioxide (CO2.) For a long time, that’s the picture scientists were working with. But recent research shows that most carbon is carried by refractory organics, which are materials with a very high sublimation temperature. In the new paper, these refractory organics are called ‘soot.’

Illustration showing one of the darkest known exoplanets – an alien world as black as fresh asphalt – orbiting a star like our Sun. The day side of the planet, called WASP-12b, eats light rather than reflects it into space. Astronomers think that the planet’s high carbon content plays a role in its low albedo. Credit: NASA, ESA, and G. Bacon (STScI)

Soots can stay solid at temperatures up to 500 Kelvin (226 C; 440 F.) And they have another important property. When they reach their destruction temperature, they decompose into simpler, more volatile species. That means their vaporization is irreversible, and that’s what leads to the soot line in a protoplanetary disk. The soot line is close to the star, and inside the soot line, refractory carbon is absent, but outside it, that critical source of carbon is available. But carbon can be tricky. “A unique property of the soot line,” the authors explain in their paper, “is that any carbon contained in the vapour that mixes outward remains in the gas and does not freeze out again as expected around traditional snow lines.”

So, planets that form outside the soot line can be rich in carbon. But planets can lose carbon while they’re still forming. Early in a protoplanetary disk, the temperature can change, and that can push the soot line further from the star. Earth’s low carbon inventory may reflect the fact that Earth gathered much of its material during this phase.

The soot line brings new complexity into our understanding of and search for potentially habitable exoplanets. This recent research shows that the frost line idea may be a bit too simple. Instead, researchers should take the soot line into account, and try to understand the carbon-rich sooty planets that can form outside it.

“It’s exciting because it leads to all kinds of endless possibilities.”

Professor Ted Bergin, University of Michigan

“For the water-poor Earth, the water ice line, or ice sublimation front, within the planet-forming disk has long been a key focal point,” the authors write in their paper. “We posit that the soot line, the location where solid-state organics are irreversibly destroyed, is also a key location within the disk. The soot line is closer to the host star than the water snow line and overlaps with the location of the majority of detected exoplanets.”

The region outside the soot line could include planets with more carbon than Earth, and that raises questions about the habitability of planets that form outside the line.

“It adds a new dimension to our search for habitability. It may be a negative dimension, or it may be a positive dimension,” Bergin said. “It’s exciting because it leads to all kinds of endless possibilities.”

As we’ve all learned in the last few years of exoplanet discovery, our Solar System isn’t representative of other systems. In other solar systems, we see planets much closer to their star than Mercury is to the Sun, and many more giant planets close to their stars. They also contain what are called Super-Earths and/or mini-Neptunes, something our system lacks.

“These are either big rocks or small gas giants—that’s the most common type of planetary system. So maybe, within all those other solar systems out in the Milky Way galaxy, there exists a population of bodies that we haven’t recognized before that have much more carbon in their interiors. What are the consequences of that?” Bergin said. “What this means for habitability needs to be explored.”

Our system doesn’t have a carbon-rich planet that formed between the soot line and the frost line, but other systems might. Unfortunately, we have no way of directly identifying these planets with current technology. So the researchers turned to models of planetary formation.

Carbon, Haze, and Habitability

Strange things happened inside the soot line when the team of researchers worked with models of planetary formation. They modelled silicate-rich worlds (the Earth is silicate rich) with 0.1% and 1.0% carbon by mass that form inside the soot line. They also varied the water content. They found that outgassing on these planets creates a methane-rich (CH4-rich) atmosphere. In our Solar System, the inner planets are methane-poor, and the outer planets are methane-rich, with Earth being the exception. Compared to its inner system siblings, Earth is methane-rich.

A methane-rich atmosphere provides an opportunity for hazes to form via interactions with protons from the Sun. We can see this happening on Saturn’s moon Titan, a methane-rich world with liquid methane on its surface.

“Planets that are born within this region, which exists in every planet-forming disk system, will release more volatile carbon from their mantles,” Bergin said. “This could readily lead to the natural production of hazes. Such hazes have been observed in the atmospheres of exoplanets and have the potential to change the calculus for what we consider habitable worlds.”

This artist’s illustration shows VHS 1256b, an exoplanet about 40 light-years away. The JWST detected methane in its atmosphere in March 2023. Image Credit: Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI)

The haze is a signal that an exoplanet contains significant volatile carbon in its mantle. Since carbon is the backbone of life, abundant carbon should affect its potential habitability. Carbon-rich haze planets that form between the soot line and the frost line might be an entirely new class of planet.

“If this is true, then there could be a common class of haze planets with abundant volatile carbon, and what that means for habitability needs to be explored,” he said. “But then there’s the other aspect: What if you have an Earth-sized world, where you have more carbon than Earth has? What does that mean for habitability, for life? We don’t know, and that’s exciting.”

But how can this research affect our understanding of exoplanets and our search for habitability?

Astronomers have detected planets with these characteristic hazes. “These hazes could readily be a by-product of birth between the soot and ice lines,” the authors write. With the advent of the James Webb Space Telescope, exoplanet scientists are poised to solidify their understanding of these hazes and what they might mean for carbon content and habitability.

“Such hazes, and the methane that drives their formation, are detectable via JWST transit spectroscopy, as demonstrated here, especially around stars lower in mass (and therefore size) than the Sun,” the authors write. “Thus, the presence or lack of hazes in the atmospheres of super-Earths or sub-Neptunes may allow us to discern whether they formed in situ from local materials or closer to the snow line and then migrated inward.”

This true-colour image of Titan, taken by the Cassini spacecraft, shows the moon’s thick, hazy atmosphere. Though the atmosphere is mostly nitrogen, it also contains about 2.7% methane, compared to Earth’s 0.00017%. Credit: NASA

For Earth-size planets, astronomers still need better models for their evolution. Carbon-poor, water-poor Earth is crowded with life. Could terrestrial carbon planets with much more carbon than Earth spawn different life on different pathways?

Carbon-rich exoplanets are known to exist. WASP 12-b has a carbon-to-oxygen ratio much higher than Earth’s. 55 Cancri e may be one-third carbon, compared to Earth’s 0.024%. In fact, 55 Cancri e might be a known example of the theoretical Carbon Planet, a planet with more carbon than oxygen.

Astronomers will keep finding more carbon-rich planets, in fact, they’ll keep finding more of every type of planet. The JWST and other future observatories will become even more adept at deciphering exoplanet atmospheres. Some day, astronomers will have the data and the models they need to understand carbon, the soot line, planet formation, and what it means for habitability.

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Evan Gough

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