ESA PLATO Mission: Goals and Strategy to Find Earth‑Like Worlds

Our solar system contains eight planets orbiting one sun, with Earth being the only one known to harbor life. However, the Milky Way galaxy is estimated to contain at least 100 billion stars, each potentially with its own orbiting planets. This raises the question of whether similar conditions for life exist elsewhere and how we can find them. The European Space Agency's (ESA) Plato mission aims to answer this.

Anna Harass, Plato's lead project scientist, and Alistair Winton, an electronics engineer, discussed the mission's objectives and technical details. They joined from ESTEC, ESA's largest center in the Netherlands.

Defining Key Terms for the Plato Mission

The Plato mission's tagline is to "discover and study terrestrial exoplanets in orbits in the habitable zone of sun-like stars." To understand this, several terms need clarification:

• Exoplanets: These are planets orbiting stars other than our sun. It is now understood that almost all stars have planets around them. Scientists are interested in exoplanets to understand the diversity of planetary systems and to contextualize our own solar system.
• Terrestrial Exoplanets: These are defined as planets with a size, mass, and density similar to Earth. This implies a composition of silicate rocks and an atmosphere that is not predominantly hydrogen or helium.
• Sun-like Stars: A star is considered sun-like if its radius, mass, and temperature are similar to our sun. This excludes stars that are significantly colder, warmer, larger, or older than the sun.
• Habitable Zone: This is the region around a star where a planet could potentially have liquid water on its surface. The distance of this zone from the star depends on the star's temperature; it's farther for warmer stars and closer for colder ones. Liquid water is considered a crucial ingredient for life as we know it.

The Cosmic Vision Program and Plato's Role

Plato is the M3 mission within ESA's Cosmic Vision program, a long-term plan initially spanning 2015-2025 but now extended for at least another decade. The program addresses fundamental questions in astronomy and planetary science, including:

• Conditions for planet formation and the emergence of life.
• The workings of our solar system and its planets.
• The laws of physics governing the universe.
• The origin, composition, and evolution of the universe.

Plato specifically contributes to understanding the conditions for planet formation and the emergence of life.

Personal Journeys to the Plato Mission

Anna Harass, an astronomer, began working on Plato even before its selection as an M3 mission. Her role involves bridging the scientific community, which proposed the mission and now comprises over 800-900 scientists, with the engineering teams at ESTEC. She helps define the scientific objectives and requirements, ensuring the mission's design and operations align with these goals.

Alistair Winton joined the project almost 10 years ago, transitioning from previous missions like Exomars and Lisa Pathfinder. He was involved in building the project teams, writing specifications, managing procurement, and negotiating with the prime contractor, OHB. His focus has been on the electrical systems, including communication systems (antennas, transmitters, receivers), onboard computers, and other electrical components.

Predecessor Missions and Plato's Advancements

Space missions often build upon previous endeavors. Gaia, an astronomy mission that mapped a billion stars in our galaxy, provides a catalog used by Plato to identify stars of interest.

More directly related to exoplanets, the French space agency's Corot mission, in collaboration with ESA, was the first dedicated exoplanet mission. Corot, using a similar method to Plato, discovered hundreds of exoplanets, including the first terrestrial exoplanet, though it was very hot and close to its star.

NASA's Kepler and TESS missions also used the transit method, with Kepler discovering thousands of exoplanets and TESS currently surveying the entire sky for more.

ESA's CHEOPS mission, part of the Cosmic Vision program, is a smaller mission that follows up on known exoplanets to determine their parameters with greater precision.

Plato aims to build on these by observing a large area of the sky with high sensitivity, allowing it to detect and characterize exoplanets around brighter stars. This enables more precise parameter determination and ground-based follow-up observations.

Plato's Advanced Camera System

Plato is equipped with 26 telescope cameras: 24 "normal" cameras and two "fast" cameras. Each camera contains four Charged Coupled Device (CCD) detectors, and each image is 81.4 megapixels. This results in a cumulative 2 billion pixel image, the highest resolution ever for a space mission.

This high resolution isn't for angular resolution but for combining a large sky coverage with sufficient sensitivity to detect small, Earth-like planets. The advanced electronics also provide a high data cadence. Unlike previous missions, Plato's ability to observe bright stars allows for better characterization of exoplanets and facilitates ground-based follow-up.

The mission will not download full 2-gigapixel images. Instead, it pre-selects hundreds of thousands of stars of interest and downloads "imagelets" – small 3x3 or 4x4 pixel areas around these stars – to manage data link capabilities.

The cameras are static, arranged in four rows of six. The entire spacecraft is steered to point at different areas of the sky. To detect the subtle dips in starlight caused by transiting planets, Plato will stare at a single area of the sky for one to two years.

The Transit Method

Plato will primarily use the transit method to detect exoplanets. This indirect method involves observing the light of a star for periodic dips in brightness. These dips occur when an exoplanet passes in front of its star from our perspective, temporarily blocking a small amount of light.

The transit method is highly efficient for finding exoplanets, especially smaller ones like Earth. It allows for the detection of very small dips in starlight, such as the 84 parts per million dip expected from an Earth-like planet transiting a sun-like star.

Advantages of the transit method include:

• High detection rate: It has yielded the highest number of exoplanet discoveries.
• Precise radius measurement: It provides an accurate measure of the exoplanet's radius.
• Orbital information: It reveals the inclination of the orbit and can indicate the presence of other planets if transit timing variations are observed.
• Mass determination (with follow-up): While not directly providing mass, it enables follow-up observations using other methods (like radial velocity) to determine mass.

Expected Discoveries and Age Determination

Plato is expected to detect thousands of exoplanets, ranging from large Jupiters to Earth-sized planets. The number of Earth-like planets around sun-like stars in the habitable zone is still unknown, with estimates varying widely (from 1% to 100%). However, if the occurrence rate is around 20-40%, Plato could discover and characterize several such planets, determining their radius, mass, and age.

Determining the age of exoplanets is crucial. Since the age of a star and its orbiting planets are typically similar, Plato will focus on measuring stellar ages. This is achieved through asteroseismology, the study of stellar oscillations. All stars oscillate due to sound waves propagating within them. These resonance modes provide information about the star's properties, including its radius, mass, structure, and age. Plato's precise observations of these oscillations will allow for accurate age determination for a large number of stars. For stars where oscillations are less detectable, asteroseismology will improve stellar evolution models, enabling better age estimates.

Electrical Testing and the Faraday Cage

A critical step before launch is electrical testing. Alistair Winton confirmed that Plato's electrical systems are safe and ready for space. This testing was conducted at ESTEC in a 9-meter-high Faraday cage, named after Michael Faraday, who first built one in 1836.

The Faraday cage, specifically the Maxwell chamber at ESTEC, is a large metallic enclosure (15m deep, 10m wide) that prevents electromagnetic radiation from entering or leaving. Its walls are also covered in absorbers to prevent reflections.

The testing serves several purposes:

1. Functionality Confirmation: To ensure the spacecraft is fully functional.
2. Launcher Compatibility: To prove to the launch authority that the spacecraft will not interfere with the launcher (Ariane 6). This involves measuring emissions from the spacecraft in its launch configuration (with only essential equipment powered on) and ensuring it doesn't disrupt the rocket's sensitive systems. Conversely, the spacecraft is radiated with the expected launchpad environment to ensure it's not susceptible to external interference.
3. Self-Compatibility (Auto-Compatibility): To ensure that once in space, the spacecraft's transmitters do not interfere with its scientific instruments, and other onboard equipment (including cameras) does not interfere with uplink communications. This is tested in a free-space environment within the chamber, with test antennas transmitting to and receiving from the spacecraft.

This electrical testing was the final environmental test. Previous tests included:

• Integration of solar arrays.
• Shaker and acoustic chamber tests: Simulating the vibrations and noise of a rocket launch.
• Thermal vacuum (TVAC) chamber tests: Simulating the extreme temperatures and vacuum of space for almost a month, testing all systems (RF communications, data handling, payload).

Launch and Data Collection Timeline

Plato is scheduled to launch in March next year from French Guiana, heading towards the L2 point, approximately 1.5 to 1.9 million kilometers from Earth.

The initial phase, called "LOP" (Launch and Operations Phase), involves the rocket taking the spacecraft into space. About 1.5 hours after launch, Plato will separate from the launcher. Within seconds of separation, it will begin transmitting, and ground teams hope to receive a signal.

The first few minutes are critical: the spacecraft will autonomously de-spin, deploy its solar wings, acquire Earth, and point towards it. This is followed by a series of commissioning tests. The journey to L2 will take about 20 days, during which the steerable antenna will be calibrated. The payload will also be switched on to ensure it survived the launch environment.

The full commissioning of the payload and spacecraft will take approximately 90 days. Low-rate housekeeping data will be received immediately after separation. As the spacecraft moves further away, higher data rates will be enabled by the high-gain antenna. Serious data volumes will begin during payload commissioning.

Plato will collect data every three months, corresponding to the spacecraft's rotation. During the commissioning and performance verification phase, it will observe known planets to calibrate its systems. Full-frame images will be downloaded periodically for calibration.

Routine operations are funded for four years. Data will be consolidated every three months. The first public data release is expected in June 2028, or possibly March 2028, after data processing and verification.

Future Studies and the Search for Biosignatures

Once Plato identifies thousands of exoplanets, follow-up studies will be crucial. An integral part of the mission involves ground-based observations using the radial velocity method to determine the mass of exoplanets, helping to confirm their terrestrial nature and rule out false positives.

For exoplanets around bright stars, the composition of their atmospheres can be studied using missions like the James Webb Space Telescope (JWST). ESA's Ariel mission, also part of Cosmic Vision, is specifically designed to determine the atmospheric composition of exoplanets around bright stars. Ground-based observatories like the European Southern Observatory's Extremely Large Telescope (ELT) with its 40-meter diameter will also contribute.

However, determining the atmospheric composition of Earth-like planets around sun-like stars to search for biosignatures (signs of life) will require the next generation of missions. These include NASA's Habitable Worlds Observatories and the proposed European LIFE mission. These missions, expected in the 2040s, aim to detect biosignatures and represent the ultimate goal for exoplanet scientists.