Earth-Like Planets

What do we know about them?

faq | July 5, 2024

Space exploration is often seen as a romantic endeavour, filled with mysteries that all can easily understand, such as the study of black holes, the search for extraterrestrial life, and the quest for Earth-like planets.

What is an Earth-like Planet?

Astrophysicists currently can measure only basic parameters of planets outside our Solar System. When we refer to Earth-like planets, we generally mean those similar in mass or size. Typically, one of these parameters is determined (around 200 such planets have been found), and in rare cases, both. These data are crucial as knowing the mass and size can lead to other inferences. If a planet closely matches Earth’s parameters in both criteria, it is almost certainly rocky, similar to Earth in composition. However, this does not provide information about other important aspects, such as the presence of water.

What Proportion of Such Planets Are in the Habitable Zone?

The concept of the habitable zone is vital for planetary research. This zone is where a planet is at a suitable distance from its star, allowing for the possibility of liquid water on its surface, assuming an atmosphere exists. These planets are the most likely to be habitable and, in turn, affect their atmospheric parameters. Several hundred planets have been found in habitable zones around various stars, including a few dozen Earth-like ones.

However, the habitable zone depends not only on the distance to the star but also on the properties of the atmosphere. Interestingly, Earth is close to the inner edge of our solar system’s habitable zone. A slight change in atmospheric parameters could push the planet out of this zone, triggering a runaway climate change that could make it uninhabitable. Thus, the issue of anthropogenic global warming is a severe crisis; even a small temperature increase can trigger a cascade of catastrophic consequences.

Scientists typically study objects relatively close to Earth. With around 300 billion stars in the Milky Way, they focus on a region containing a few million of the nearest stars. Consequently, most discovered Earth-like planets in habitable zones are astronomically close to us. Finding an Earth-like planet in the habitable zone at the far end of our galaxy is possible but very challenging.

What Methods Are Used to Study Earth-Like Planets?

Currently, two main methods are used to find Earth-like planets. The first is the radial velocity method. Although we can’t see the planet itself, we can study the star it orbits. Planets and stars orbit a common centre of mass. By observing the star, scientists can measure its speed and movement period—toward and away from us—and determine the planet’s orbital period, mass, and distance from the star.

The second and most effective method today is the transit method. Once per orbit, a planet may pass directly between the observer and the star, causing a slight dimming of the star’s light due to the planet’s shadow. Satellites can detect this dip in brightness, leading to the discovery of thousands of planets, thanks to satellites like Kepler and TESS.

Once a planet is discovered using this method, its atmosphere can be studied. Different wavelengths reveal different levels of atmospheric transparency. Measuring the drop in starlight during transit can provide new information, such as the presence and composition of an atmosphere. However, this method is currently more applicable to larger objects or planets close to their stars. Studying Earth-like planets in habitable zones will become feasible with next-generation telescopes.

One major challenge remains: studying the atmospheres of Earth-like planets requires multiple transits, but an ideal transit happens only once a year. Thus, years of data accumulation are needed. By around 2040, new types of space telescopes may solve this problem directly without lengthy data collection.

Can Life Exist on Planets Outside the Habitable Zone?

Within our Solar System, scientists primarily search for life on objects outside the habitable zone, like the moons of major planets: Europa, Enceladus, Ganymede, and Titan. These moons are covered with ice except for Titan, and life may be found in subsurface oceans. Beyond our Solar System, life will likely be detectable through atmospheric composition. For exoplanets, life must be on the surface, affecting atmospheric parameters, rather than hidden under ice.

Interestingly, scientists search for life based on what they know about Earth. They analyze atmospheric composition using data from Earth’s current atmosphere. Earth’s life appeared relatively recently, and 3 billion years ago, it existed only as bacteria, with an atmosphere different from today’s. Knowledge of Earth’s ancient atmosphere is crucial for scientists seeking alternative life. Thus, discoveries in Earth’s palaeoclimatology can significantly influence the search for extraterrestrial life.

How Often Do Discoveries in the Search for Earth-like Planets Occur?

Discoveries often follow the launch of new space missions. The Kepler telescope, during its four-year mission, discovered about 2,600 planets. The TESS telescope’s primary mission lasted two years, yielding significant results. The next major mission is expected in 2027 with the launch of the PLATO satellite. New atmospheric data will come from the Extremely Large Telescope at the European Southern Observatory, set to begin operations in 2025.

However, significant discoveries also occur outside large programs. For instance, Proxima Centauri B, the nearest known exoplanet, was discovered through individual research. Such discoveries happen roughly every few years.

Associate research officer, ICTP, Trieste
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