Astrobiologist's Quest to Crack the Code of Exoplanet Life

With a theoretical model of photochemistry, Antígona Segura Peralta, an astrobiologist at UNAM's Institute of Nuclear Sciences (ICN), performs simulations through numerical codes to study what life on an exoplanet (a planet outside the solar system) could be like.

"They have a series of equations that come out of physical laws or chemical reactions. My models in particular simulate how the radiation comes in and how all the chemical reactions are happening because of the radiation from the star, assuming certain things."

"For example, I'm assuming that my planet has an atmosphere of carbon dioxide and nitrogen, as the Earth did in the beginning, as do Mars and Venus, and then I'll see what happens when I inject an amount of ultraviolet radiation from one of these stars. I see what chemical reactions happen, and that allows me to predict what things I can see on planets around other stars," she said.

We understand very well that life can form from totally inorganic compounds, such as methane, carbon dioxide, hydrogen, and nitrogen, which are surely available on other planets, as they were on Mars, for example.

Some metals are also required, such as phosphorus, and from this, the molecules begin to organize. "There were also completely random processes based on things that were in the environment," said the university professor.

The particulars of exactly how the first cells originated are still obscure, but we have a clear idea that from certain geological situations and raw materials, life emerged on Earth, added the expert.

"These geological contexts could happen on other planets because planets made of rocks, like Earth, could happen on other worlds. The question is that we don't know with what probability," she said.

The scientist pointed out that knowing if life exists on other planets, even exoplanets is one of the open questions of science, especially astrobiology, which studies the origin, evolution, distribution, and future of life in the universe.

"Our idea of planetary habitability is based on terrestrial life, which in general requires carbon chemistry and liquid water," Segura Peralta summarized.

In an interview, he specified that carbon is a chemical element that can join up to four atoms at the same time, including itself, which is capable of forming large structures that can eventually have other complex structures. "For these molecules to meet, react, and accumulate, a liquid medium is needed, which on Earth is water."

She said that if we think about the places where planets form, which are the molecular clouds in which material from the last phases of stars accumulates, carbon and water are also common.

"Planets around M dwarf or red dwarf stars are potentially habitable, is part of the scientific discussion. The problem with them is that they have chromospheric activity, which is that they emit large amounts of X-ray radiation and ultraviolet radiation, and sometimes suddenly there is a spike in all of this. They have high X-ray radiation, and this could be harmful to life, so there is a debate," she explained.

The planets are very close to these M dwarf stars, which, being less massive, are not very luminous, with disks where the low-mass planets form. "The habitable zone, which is this place where potentially habitable planets could exist, can be sampled with the instruments we have now," the scientist added.

For example, if we think of the Earth and the Sun, which are one astronomical unit away, we would need to sample at least two years before observing our planet either by the transit method or by the radial velocity method, which is the two most common.

Detecting them in the habitable zone of stars like the Sun is technically more complicated than doing so around M dwarf stars. That is what has made them good candidates to search for, but they have their problems as well, Segura Peralta said.

To investigate habitability, the expert uses photochemical models, but other colleagues in the world calculate the temperature of the atmosphere and the atmospheric escape, that is, what happens if the atmosphere disappears, which is very important for habitability because it allows maintaining liquid water on the surface. In general, these are theoretical models.

There are also experimental ones in which a mixture of gases is put in a flask and subjected to different energy sources. Other experiments are carried out with extremophile bacteria, which can survive in different environments, to test the limits of life.

Segura Peralta said that so far, the only mission that has specifically searched for life on another planet was Viking in the 1970s. "It scraped and took a sample of Martian soil and then analyzed it in experiments that were made to detect biological activity."

For planets around other stars, we can only investigate with off-Earth telescopes because the atmosphere gets in the way and it's like looking through a swimming pool, she said.

Now with the James Webb space telescope, the most advanced telescope in space, we can detect the atmospheres of some potentially habitable exoplanets and establish if there are signs of habitability, which would be carbon dioxide and water. And then indications that the planet may be inhabited. "This is the first mission with the capability to do that, even if it's limited."