While we have made remarkable progress in astronomy, we are still far from answering these profound questions. However, two upcoming ESA (European Space Agency) missions are set to bring us significantly closer to uncovering the truth.

In mid-March, SRON—the Netherlands Institute for Space Research—will host the PLATO-Ariel Community Day at their institute in Leiden, bringing together the Dutch exoplanet community. This workshop will provide updates on the PLATO and Ariel missions and conclude with a strategic discussion among Dutch scientists. If you are not an astronomer or space enthusiast, you might not have heard of these exciting upcoming ESA missions: PLATO, set to launch as soon as 2026, and Ariel, scheduled to follow in 2029.

Brief excursion into exoplanet detection and planet types

But let us start from the beginning to make sure we are all on the same page: What exactly is an exoplanet? An exoplanet is simply a planet that is not part of our Solar System. It could either be orbiting another star or be freely floating through space. Most of these distant worlds are unimaginably far away—the closest known one, Proxima Centauri b, is still about four light-years from Earth. Astronomers detect exoplanets using a variety of techniques, with the transit method currently being the most widely used. This method involves observing a star over an extended period and looking for periodic dips in its brightness. When a star’s light dims at regular intervals, it suggests that an object is passing in front of it, temporarily blocking a small fraction of its light. In most cases, that “dimming object” is a planet.

There are different types of planets, as we can see from our own Solar System. A planet’s classification depends on its mass and size. Starting with the smallest and lightest ones, Mercury, Venus, Earth, and Mars are known as terrestrial or rocky planets. These planets are Earth-sized or smaller and are composed primarily of rock and metal. Beyond our Solar System, astronomers have discovered many planets that do not have counterparts here. One such class is super-Earths—planets more massive than Earth but lighter than Neptune. Like terrestrial planets, super-Earths are typically rocky and may have diverse environments.

Some of these terrestrial exoplanets show promising signs of potential habitability, such as the presence of oceans, atmospheres, or other conditions that could support life. A habitable environment is generally one that could potentially sustain life of any form. For terrestrial exoplanets, the key requirement for habitability is the long-term presence of liquid water on the planet’s surface—a critical ingredient for life as we know it. The habitable zone (HZ) can then be understood as a region around the star, where water could possibly be found in its liquid form on the surface of a planet with an Earth-like atmosphere orbiting there. This depends on the amount of radiation that the planet receives from the star.

Continuing with the heavier planets out there, planets similar to Neptune and Uranus, characterized by their extended hydrogen-helium atmospheres, are simply referred to as Neptunes. Another fascinating class that can also not be found in our Solar System, is the mini-Neptunes. As the name suggests, these planets are smaller than Neptune but larger than Earth. Typically they feature hydrogen- and helium-dominated atmospheres surrounding rocky or icy cores. However, when astronomers discover an exoplanet, it is often challenging to determine whether it should be classified as a super-Earth or a mini-Neptune, as the distinction between the two categories is not always clear.

At the upper end of the spectrum are the gas giants, the largest and most massive planets. These giants are similar to—or in some cases even more massive than—Jupiter and Saturn, with vast, swirling atmospheres dominated by hydrogen and helium.

PLATO and Ariel

Now that we have a better understanding of the different types of exoplanets and how they are detected, the question arises: how do we move from simply finding these distant worlds to truly understanding them? This is now where the PLATO and Ariel missions come into play. Designed to not only discover new exoplanets but also to characterize them in unprecedented detail, these upcoming ESA missions will help us get closer to answering the age-old questions about our and Earth’s uniqueness.

PLATO: Exploring Planetary Systems and Habitability

PLATO stands for PLAnetary Transits and Oscillations of stars. Its main scientific goals are to address fundamental questions such as:

• Is our Solar System truly unique, or are there other systems like ours?

• Are there potentially habitable planets beyond Earth, or is habitability an Earth-exclusive feature?

• How do planets and entire planetary systems form and evolve?

To answer these questions, the PLATO consortium will detect and characterize thousands of terrestrial exoplanets orbiting sun-like stars within their habitable zones. As suggested by its name, the PLATO satellite will primarily rely on the transit method to detect these planets. By analyzing data from other detection methods using additional ground-based telescopes, astronomers can then determine key planetary and system parameters, such as a planet’s radius, density, and the age of its host star. These characteristics are crucial for assessing the habitability of newly discovered worlds.

Ariel: Unveiling Exoplanet Atmospheres

Ariel, short for Atmospheric Remote-sensing Infrared Exoplanet Large-survey, represents the next step in exoplanet research. While PLATO focuses on discovering and characterizing new planets, Ariel is designed to shift our focus from discovery to understanding. Its primary goal is to study the chemical composition of exoplanetary atmospheres through remote sensing in the infrared spectrum.

Remote sensing means acquiring information from a distance—since we cannot physically visit these distant worlds. The infrared part of the spectrum is key because many atmospheric molecules, such as water vapor and carbon dioxide, strongly absorb light at these wavelengths. These absorption features appear as dips in the spectrum of light we detect from exoplanets, providing insights into their atmospheric composition.

In addition to identifying atmospheric chemicals, Ariel will explore how a host star influences the conditions on its orbiting planets. For example, if Earth orbited a hotter star, much of its surface water would evaporate, making it uninhabitable.

Together, the discoveries of PLATO and Ariel will undoubtedly help us take the next step in our search for the diversity of planets out there and potentially habitable worlds. And who knows—perhaps we will even uncover an Earth 2.0.

Sources

Many thanks to Dr. Y. Miguel for fact-checking

Inspiration:

SRON Netherlands Institute for Space Research. (2025, 27 januari). The PLATO-Ariel Community Day - SRON Netherlands Institute for Space Research. SRON Netherlands Institute For Space Research. https://www.sron.nl/the-plato-...

Image information source:

Santos, I. D., Valio, A., Omar, N., & Guimarães, A. (2016). Exoplanet Classification with Data Mining. Journal Of Computational Interdisciplinary Sciences, 7(3). https://doi.org/10.6062/jcis.2...

1. Plato. (z.d.). https://www.esa.int/Science_Ex...
2. Ariel factsheet. (z.d.). https://www.esa.int/Science_Ex...
   
3. Tran, L., & Tran, L. (2025, 30 januari). Exoplanets - NASA Science. NASA Science. https://science.nasa.gov/exopl...
   
4. Cermak, A. (2024, 29 oktober). Overview - NASA Science. NASA Science. https://science.nasa.gov/exopl...
   
5. PLATOMission. (2018, 7 juni). Habitability of planets around solar-like stars. https://platomission.com/2018/...
   
6. Cermak, A. (2024a, oktober 28). What Is a Super-Earth? - NASA Science. NASA Science. https://science.nasa.gov/exopl...
   
7. Remote sensing. (2024, 29 oktober). NASA Earthdata. https://www.earthdata.nasa.gov...