Exoplanets: On the Trail of Earth 2.0

Exoplanets: The Quest for Habitable Worlds

“Are we alone in the Universe?” This question has long been on the minds of astronomers. Answering it requires knowing whether other planets exist where other forms of life could develop, hence the search for exoplanets.

Worlds Beyond Our Immediate Reach

Exoplanets, by definition, are located outside our solar system. With several hundred billion stars in the Milky Way alone, the search for these extrasolar planets is far from over, even though it remains confined to our own galaxy: with the exception of a few punctual detections by gravitational microlensing in nearby galaxies, almost all known exoplanets orbit stars in the Milky Way, as intergalactic distances make their individual detection beyond the reach of current instruments.

Detecting such planets is not easy. They are very far from us, emit no light of their own, and the brightness of the star they orbit literally drowns them out. The Hubble Space Telescope can only observe Pluto, a member of our own solar system, as a barely resolved point. How, then, can we hope to distinguish an extrasolar planet orbiting a star sometimes thousands of times farther away than Pluto?

The Gravitational Trick: When the Star Betrays Its Planet

Earth orbits the Sun, but in reality, if a star is accompanied by a single planet, both orbit around the center of mass of the gravitational system they form. Given the star's much greater mass, this center of mass is much closer to the star than to the planet. It is even most often inside the star, without, however, coinciding exactly with its geometric center.

The star thus undergoes a slight periodic displacement caused by the presence of the planet. This is precisely what the radial velocity method reveals: it consists of detecting these variations in the spectrum of the light emitted by the star, via the shift of its spectral lines.

The Doppler Effect: The Colored Sign of an Invisible Presence

This spectral shift is explained by the Doppler-Fizeau effect: the star's light appears slightly redshifted when it moves away from the observer and blueshifted when it approaches. As the star is alternately pulled in one direction and then the other by the orbiting planet, its spectrum periodically oscillates between a slight excess of red and a slight excess of blue, at the rhythm of the planet's orbital period.

This alternation, although tiny—on the order of a few meters per second for an Earth-sized planet—can be measured with very high-precision spectrographs like HARPS or ESPRESSO. It enabled the very first detection of an exoplanet around a solar-type star, 51 Pegasi b, in 1995, and remains today one of the reference methods for confirming and characterizing worlds discovered by other techniques.

Exoplanet Detection Techniques in the Milky Way

Since the discovery of the first exoplanet around a solar-type star in 1995 (51 Pegasi b), more than 8,000 worlds have been confirmed beyond the Solar System (as of February 2026). Five major methods, often complementary, allow astronomers to detect and characterize them.

1. Radial Velocity Method (Doppler Spectroscopy)

• Measures the periodic motion of a star around the common center of mass with its planet, under the effect of the planet's gravitational attraction
• Detects this motion via the Doppler shift of the star's spectral lines, alternately redshifted and blueshifted
• Provides access to the orbital period and a minimum mass (m·sin i), with the system's inclination generally remaining unknown

Limitation: Sensitivity decreases with distance from the star and the planet's mass; the method thus favors the detection of massive planets on short orbits ("Hot Jupiters").

2. Transit Method

• Detects the tiny drop in a star's brightness when a planet passes in front of its disk
• Allows the planetary radius to be deduced from the depth of the transit
• Combined with transmission spectroscopy, it allows the composition of planetary atmospheres to be analyzed (notably with JWST)

Limitation: Only works if the planet's orbital plane is almost aligned with the line of sight; only a fraction of planetary systems are favorably oriented.

3. Direct Imaging

• Involves directly photographing the light emitted or reflected by the planet, separated from that of its star
• Uses coronagraphs or interferometric masks to suppress stellar glare, combined with adaptive optics
• Provides direct access to the spectrum, brightness, and sometimes the orbit of the planet

Limitation: Reserved for young, massive, hot planets widely separated from their star; in 2025, JWST made its first unprecedented discovery by coronagraphy, a low-mass planet in the debris disk surrounding the young star TWA 7.

4. Gravitational Microlensing

• Exploits the deflection and magnification of light from a background star by the gravitational field of a lens star and its planetary entourage, in accordance with general relativity
• Sensitive to planets at great distances, including toward the galactic center
• Only method capable of revealing rogue planets, not gravitationally bound to a star

Limitation: Transient and non-reproducible event, which never repeats for the same system; in July 2026, the TESS telescope—designed for transits—made its first microlensing detection (Gaia23bra b) by re-examining its archives following a Gaia alert, proving that the method can also be applied a posteriori to data collected for another purpose.

5. Astrometry

• Measures the tiny periodic displacement of a star in the plane of the sky, caused by orbital motion around the common center of mass with its planet
• Particularly sensitive to massive planets on wide orbits, unlike radial velocities
• Complementary method to others, notably to resolve ambiguity about inclination and thus the true mass of planets

Limitation: Requires extreme measurement precision, on the order of microarcseconds; the ESA's Gaia mission (2013-2025), whose DR4 catalog is expected in 2026, should provide the first major astrometric survey of exoplanets.

These methods, often used in combination, have enabled the discovery of more than 8,000 confirmed exoplanets and reveal a diversity of planetary systems much greater than what the sole example of the Solar System suggested.

The Hunt for Exoplanets

A Historically Solar-Type Star-Centered Method

The radial velocity method, which enabled the very first detection of an exoplanet around a solar-type star in 1995 (51 Pegasi b), consists of detecting the periodic motion of a star around the center of mass it shares with its planet. It is not the star itself that "oscillates" in the proper sense, but its position varies slightly under the effect of the gravitational attraction of the orbiting body, a displacement revealed by the Doppler shift of its spectrum.

• Historically targets solar-type stars (spectral class G), which are easier to model and well-characterized spectrally
• Detects the gravitational perturbation exerted by one or more planets on the motion of their host star
• Measures periodic variations in the star's radial velocity, i.e., its speed along the line of sight

A Detection Bias Favorable to Hot Giants

This method does not "miss" small rocky planets by chance: it suffers from an intrinsic detection bias that makes it much more sensitive to gas giants in close orbits than to terrestrial planets.

• "Hot Jupiters"—gas giants in very close orbits to their star—are the most easily detectable
• A high planetary mass induces a greater displacement of the star, hence a more marked Doppler signal
• A close orbit shortens the orbital period, allowing the signal to be confirmed over several cycles in a short time

The Importance of Giants

The Role of the Protoplanetary Disk

The amount of dust and gas available in the protoplanetary disk surrounding a young star largely determines the size and number of planets that will form there. According to the core accretion model, gas giants form preferentially beyond the snow line, where the condensation of volatile compounds provides enough solid material to rapidly assemble a massive core, capable of later capturing a gas envelope before the disk dissipates.

• The total mass of the protoplanetary disk strongly conditions the architecture of the future planetary system
• A more massive disk favors the formation of larger planets, particularly gas giants
• Gas giants form mainly in the outer, cold regions of the disk, beyond the snow line

Multi-Giant Systems and Post-Formation Dynamics

When the protoplanetary disk is particularly rich in gas and dust, several giants can form successively, giving rise to multi-giant systems like that of HR 8799, which has four giant planets.

• The presence of multi-giant systems generally testifies to a particularly massive protoplanetary disk
• These multiple systems durably shape the orbital architecture of all the planets, including the smallest ones
• Gravitational interactions between giants can destabilize orbits, migrate planets, or eject some of them from the system

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