JWST detected carbon dioxide and methane in its atmosphere.
By using the James Webb Space Telescope, an international
team has just detected the presence of carbon dioxide and methane on the
exoplanet K2-18 b. This world — which is 120 light years from us — is unlike
anything in our solar system. It has greater mass than Earth, but less than
Neptune.
These types of planets potentially have hydrogen-rich
atmospheres and surface areas covered in oceans of water. That’s why they’ve
also been called hycean (an acronym for hydrogen and ocean). However, since we
have nothing similar nearby to compare to these exoplanets, we don’t know them
well enough.
The atmospheres of the hycean planets are relatively simple
to characterize. But, still, we have to use the infrared James Webb Telescope
that is one million miles from the earth — complemented with data obtained from
the Hubble Telescope and various ground-based telescopes —, and even then we’re
not sure whether these exoplanets can harbor life. While deciphering the
mysteries of the universe is fascinating, nobody said that it was easy.
The abundance of methane and carbon dioxide and the scarcity
of ammonia – all detected in K2-18b (which is eight times larger than Earth) –
support the hypothesis that it may contain an ocean of water under a
hydrogen-rich atmosphere. These observations – which are currently being
clarified with more data – also speak of a possible signal from a molecule
called dimethyl sulphide (DMS). The signal would have to be confirmed, but on
Earth, this molecule is only produced by marine phytoplankton. Scientists urge
caution before discussing signs of life.
Both the origin and the evolution of life on our planet are
written in the simple carbon atom. It could be said that this chemical element
is the basis of the building material that makes up our bodies. But not only
that: carbon controls global climate cycles, something that our energy
dependence on fossil fuels is altering. Furthermore, it’s a fundamental
component of the materials in what our daily technological life is increasingly
based on.
 |
Spectrum of the atmosphere of the planet K2-18 b, according
to data gathered via the James Webb Space Telescope NASA, CSA, ESA, J. Olmstead (STScI), N. Madhusudhan
(Cambridge University) |
Oftentimes, in astrophysics, when we talk about the
discovery of new exoplanets, we mention the habitable zone. We define this as a
region with a simple characteristic: the possibility of liquid water existing
on the surface of the planet. This simple parameter that is related to the
distance to the star helps us evaluate the possibility of the existence of
extraterrestrial life. This is because of a complex but very revealing
observation: on our planet, life needs liquid water, and it is the only place
where it has been shown to exist.
Those who know a little more about astrobiology can always
argue that this is a very simplistic definition and that it’s not enough to
define a habitable zone. This is true. However, as we move away from the Earth,
we lose information. Therefore, we have to simplify the parameters that we use
to determine the possibility of life. Of course, new telescopes – such as the
James Webb – help us not only refine the models, but also make measurements
that, just a few years ago, we thought were impossible.
The temperature of the Earth’s surface – one of the most
important physical variables in what we broadly call “climate” – is controlled
by a series of factors closely linked to each other: solar insolation, albedo
and the content of greenhouse gases. What we call solar insolation is what
tourists from cold countries learn when they visit the coasts of a sunny
nation: simply, the amount of energy that the planet receives from the Sun. It
basically depends on changes in luminosity (energy per second over the entire
range of wavelengths) emitted by the star and the distance at which the planet
is located.
Albedo is simply the part of that energy from the Sun or
star that a planet – as a whole – reflects back into space. It’s easy to see
how albedo is influenced by clouds, ice layers, vegetation, the amount of land
versus ocean on the planet and aerosols (that set of microscopic particles,
solid or liquid, that are found suspended in a gas). And all of this,
furthermore, varies on different time scales. Hence, climate models are very
complex.
The part of the Sun’s energy that isn’t directly reflected
by the albedo and returned to space is absorbed by the Earth’s surface and
radiated again, in the form of infrared energy. And this is where greenhouse
gases come into play. They absorb that infrared radiation, with the overall
effect of warming the atmosphere. Thus, if greenhouse gases fall below a
certain limit, liquid water could freeze throughout the planet (these gases
aren’t bad, in moderation).
On the other hand, if that threshold is exceeded, most of
the water turns into vapor, which must have happened to Venus. For most of our
planet’s geological history, the dominant greenhouse gas has been carbon
dioxide. It has been demonstrated – with experimental measurements – that its
concentration is regulated by a kind of internal thermostat, which has allowed
the existence of a moderate climate for more than a billion years. It’s also
firmly demonstrated that all this is in the process of changing on Earth in the
coming years, due to the human injection of this gas into the atmosphere,
coming mainly from the burning of fossil fuels.
Stating, therefore, that a planet is habitable without
having measurements of its atmospheric composition is a gross simplification in
astrophysics and we know it. For this reason, all planetary characterization
efforts are focused on the detection of the molecules that dominate atmospheric
chemistry, such as carbon dioxide, methane, ozone and water. This is also the
reason why we need to know precisely how much energy the star emits in the
entire range – not just in the visible range. And that’s why we’re afraid of
losing the Hubble Telescope, because at the moment, it’s the only working
instrument that allows us to measure the energy of stars in the ultraviolet
range.