Mar 12 2021

Planet with Secondary Atmosphere

Published by under Astronomy
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The discovery and exploration of exoplanets over the last three decades has been an exciting addition to astronomy. In 1990 we knew of no planets outside our solar system, and now there are more than 4,000 confirmed exoplanets, and thousands of more candidates awaiting confirmation. This is still just a tiny sample of the planets even in our small corner of the galaxy. One of the questions going into this enterprise was – how typical are the planets we know in our own system, and also how typical is our system in terms of the number and arrangement of planets? So far the answer seems to be that there is no typical. We are finding all kinds of planets in all kinds of arrangements.

We can now add, potentially, one additional planetary phenomenon to the list – a planet with an apparent secondary atmosphere. The planet is GJ 1132 b and is 41 light years away (in naming convention the star itself has the designation “a” and so “b” is the first planet discovered in the system). This is a red dwarf star, and the planet is very close, so close that it is tidally locked, meaning the same side faces the star at all times. It’s year is only a day and a half. Planets this close to their parent stars will tend to have their atmospheres stripped by the heat and solar wind from the star.

Astronomers believe that GJ 1132 b was once a subNeptune – a planet with a rocky core about the size of Earth, but a thick hydrogen-helium atmosphere making it a gas giant. But soon after formation that atmosphere would have been blown away, leaving behind the rocky core. So astronomers expected to see little to no atmosphere on GJ 1132 b, and instead they find evidence of an atmosphere about as thick as Earth’s. This and other evidence has led them to believe this is a secondary atmosphere.

Stars tend to be hotter when they are very young and then cool down a bit. Red dwarfs change even more. They are unstable when young, giving off lots of coronal mass ejections, constantly blasting any nearby planets. This does not bode well for the prospect of life on any such planets. Unfortunately, any planet in the habitable zone is also in this blast radius and would have its atmosphere stripped. Red dwarfs are the most common star type in the galaxy, making up 70% of the stars, so this has implications for the probability of life in the galaxy.

The question then becomes – is there a way for a planet in a red dwarf system to somehow be habitable despite these obstacles. One hypothesis is that a planet can start its life further from the star then slowly migrate in to the habitable zone, and not get there until after the star has settled down. We know that this kind of planetary migration can happen, but getting all the variables just right means it is probable a rare event. There is also the issue of, what is the perfect distance from the star. You want to be in the zone, but not so close that you will be tidally locked. Although, a tidally locked planet might not be incompatible with life. There may be a zone on the planet where temperatures are just right, and this may be bigger than you might at first assume if there is an ocean and atmosphere that can efficiently move heat from the hot side to the cold side.

Now, maybe, there may be another way to allow a red dwarf star to have a planet with life – what if, after the star settles down, the planet reconstitutes its atmosphere? That would do it, but how would that work.

On GJ 1132 b astronomers believe at this point that the secondary atmosphere comes from volcanic activity. That would have to be a lot of volcanic activity, however. The atmosphere contains hydrogen, methane, and other hydrocarbons (think smog), so would not be comfortable for Earth life, but hydrogen or methane breathers are theoretically possible. Astronomers further think that the hydrogen is coming from deep within the mantle where it was captured when the planet had its thick hydrogen atmosphere. Further, they think that tidal heating in keeping the planet molten and active, with gases leaking through cracks in the thin crust.

All of this makes GJ 1132 b a terrible candidate for life as we know it. But the question is – are conditions like this necessary for a secondary atmosphere? Would less volcanic activity, with a thicker and more stable crust, suffice? Did there have to be a thick gas giant type atmosphere first? In other words, is there a scenario in which a planet in the habitable zone of a red dwarf could reconstitute a secondary atmosphere later in life in conditions that are comfortable for water-based life?

We still have a lot to discover. Trying to find habitable worlds around red dwarfs may be a lost cause, but not all hope is lost. Still, orange stars may be our best best for habitable Earth-like planets. They are warm enough, with a long life-span and generous habitable zone at a safe distance from the star. Yellow stars, like our own, are also a good bet, there is just many times as many orange suns and they last longer. Chances are, most of the habitable worlds we find will be around orange stars. But I have not lost all hope for red dwarfs.

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