In 2020, phosphine was discovered on Venus, and since then it has been considered evidence that life is possible there. However, this same molecule was recently discovered in the atmosphere of a brown dwarf, where life is definitely impossible. This means that there is a way to synthesize it without the involvement of living beings.
Mysterious molecule in the atmosphere of a brown dwarf
In a new study published in the journal Science, a team of astronomers reports the discovery of a strange substance in a brown dwarf known as Wolf 1130C: a chemical compound called phosphine, which has been the subject of controversial claims about evidence of life on Venus.
However, the presence of phosphine in the turbulent and inhospitable atmosphere of a brown dwarf shows that our understanding of the life cycle of this small, simple molecule is incomplete — and calls into question the idea that it can be considered a “biosignature” of alien life.
Nature of brown dwarfs
Like real stars, brown dwarfs form from collapsing gas clouds in space. Gas heats up as it moves inward, but in a brown dwarf, it never reaches a sufficient temperature to initiate hydrogen fusion in the star’s helium fuel. But if the gas cloud weighs at least 13 times more than Jupiter, it will heat up enough to synthesize a slightly heavier form of hydrogen called deuterium. This fusion will quickly burn out in astrophysical terms, after 1 to 100 million years.
However, gravitational collapse and nuclear fusion generate enormous amounts of heat in the core of a brown dwarf. This creates a convection cycle: gas near the core heats up and rises, transferring heat to the upper levels, then cools and sinks back down. Brown dwarfs are significantly cooler than stars. The surface of the youngest and highest ones can reach 2000°C, but the coldest ones are close to room temperature. When heat from the core reaches the surface layers of a brown dwarf, it is emitted into space mainly in the form of infrared photons.
Infrared wavelengths are difficult to observe with ground-based telescopes, but space telescopes such as the James Webb Space Telescope provide a better view. This allows us to observe brown dwarfs better.
Chemical role of phosphine
Interesting chemical reactions and processes occur in brown dwarfs, whereas this is not observed in hot stars. For nearby brown dwarfs, JWST can monitor the consequences of these chemical reactions. This is done by searching for the “barcodes” of each molecule, specific patterns of dark lines in the spectrum of light emitted by the brown dwarf. Phosphine is a simple molecule consisting of one phosphorus atom and three hydrogen atoms. In 2020, some scientists believed they had detected its spectral signature in the atmosphere of Venus. Conditions on Venus mean that phosphine should break down quickly there, so its detection would mean that something is producing large amounts of phosphine.
On Earth, phosphine is found only through life, so it has been actively studied as a possible sign of life.
But if we delve deeper than the headlines, the situation becomes more nuanced. Phosphine has been found in the atmospheres of Jupiter and Saturn, and no one is suggesting that life exists in the clouds of these planets. The reason is that we understand how phosphine can form and survive in the lower atmosphere of these planets. Then, it rises to the surface, where it quickly breaks down, but not before we see its spectral signature.
Phosphine in the atmospheres of brown dwarfs
According to some models, we should see significant amounts of phosphine in the atmospheres of brown dwarfs and hot Jupiter exoplanets. However, preliminary observations by JWST, which examined 23 brown dwarfs with temperatures ranging from 100°C to 700°C, did not detect phosphine.
The latest observation of phosphine on the brown dwarf Wolf 1130C (temperature approximately 320°C) fits the models very well. Why? The researchers behind the new study are not yet able to provide a definitive answer. Their best guess is that this may be due to the fact that Wolf 1130C is old and contains low concentrations of metals.
At this stage, they draw a simple conclusion: there is no consistent model that explains the amount of phosphine we observe on Jupiter, Saturn, Wolf 1130C, other brown dwarfs, and the atmospheres of gas giant exoplanets. Without a better understanding, the use of phosphine as a biomarker is questionable.
So, perhaps there was phosphine on Venus after all, but it was caused by unknown chemistry or physics, not biology. Extraterrestrial life remains a hypothesis of last resort.
According to phys.org
