Are there other planets that support life outside of Earth? Current research has found that…

The universe, this vast and mysterious realm, has inspired human beings' infinite curiosity since ancient times. Are we alone in this sea of ​​stars? Are there other planets that nurture life besides Earth? These questions have always been the focus of astronomers' exploration.

In 1995, two Swiss astronomers, Michel Mayor and Didier Queloz, discovered a planet with an orbital period of 4.2 days around the star 51 Pegasi using the radial velocity method. This discovery not only opened a new era of astronomy, but also provided us with a new perspective for exploring the mysteries of the universe. This pioneering work won them the 2019 Nobel Prize in Physics, highlighting the importance of exoplanet research.

Didier Queloz (left) and Michel Mayor (right)

(Image source: ESO)

What is an exoplanet

Exoplanets refer to planets outside the solar system, and are called extrasolar planets or exoplanet in English. The term "planet" defined by the International Astronomical Union (IAU) in 2006 only includes the solar system, so it does not apply to exoplanets. However, the International Astronomical Union also has a definition of exoplanets, which was promulgated in 2001 and revised in 2003. It is stated as follows: A celestial body whose true mass is lower than the lower limit of mass required for deuterium nuclear fusion (for celestial bodies with similar metallic abundance to the sun, this mass is 13 times the mass of Jupiter) and which revolves around a star or stellar remnants is called a "planet". Its mass/scale lower limit is the same as the mass/scale lower limit of the definition of planets used in the solar system.

Regardless of the formation method and location, substellar objects whose actual mass exceeds the lower limit of mass required for deuterium nuclear fusion are called "brown dwarfs". Free-floating objects in young star clusters whose mass is lower than the lower limit of mass required for deuterium nuclear fusion are not called "planets", but "sub-brown dwarfs" (or other more appropriate names). In fact, with the continuous discovery of new exoplanets, this definition has also revealed its limitations. Some astronomers suggest distinguishing exoplanets from brown dwarfs and sub-brown dwarfs based on the mechanism of planet formation.

Exoplanet Observation Methods

With the advancement of technology, humans have mastered a variety of methods to search for exoplanets. The following are some mainstream methods:

1. Radial Velocities (RV) is also called Doppler spectroscopy.

This method relies on observing stars in the spectrum to look for signs of "wobbling". Due to the influence of exoplanets on their host stars, when they move to different positions, the exoplanets will pull the host stars in different directions, causing this "wobbling" phenomenon.

Essentially, the radial velocity method is not about looking for signs of the planet itself, but about observing signs of the motion of the star. The spectrum is used to measure the way the spectral lines of the star are shifted due to the Doppler effect, that is, how the light of the star changes in the direction of the spectrum (redshift/blueshift). These changes indicate that the star is moving away (redshift) or turning (blueshift) towards the earth. Based on the speed of the star, astronomers can determine the existence of a planetary system and calculate the mass of the planet. Before the large-scale application of the transit method, it has always been the most important means for humans to explore exoplanets.

Principle of Radial Velocity Method

2. Transit Photometry, also known as the transit method or eclipse method, is an observation method that analyzes changes in star brightness when a transit occurs to infer the orbit and parameters of a planet.

Exoplanets are usually hidden in the light of their parent stars and cannot be observed directly. The transit method is a way to indirectly infer the existence of exoplanets. If a planet passes in front of the disk of its parent star (a process called a "transit"), the visual brightness of the star will be observed to decrease slightly. The degree of dimming depends on the size of the planet relative to the star. For example, if the parent star is the size of the sun and the planet is the size of Jupiter, when the planet blocks the parent star, the light curve of the star will produce a decrease of about 1% when observing this transit process from the earth. In other words, this is equivalent to a decrease in its brightness of 0.01 magnitude. If it is a terrestrial planet, then the amplitude will be smaller.

The transit method is currently the most widely used method for observing exoplanets, but it also has its limitations, because the transit method can only determine the radius of a planet but not its mass. The candidate objects discovered by the transit method still need to use the radial velocity method to confirm their mass.

Transit Method Principle

3. Direct Imaging is also called direct imaging. As the name suggests, it is to directly image an exoplanet. However, it is very difficult to directly take an image of a planet because the planet reflects too few photons and is covered by the star. This requires the planet to be large enough and not too close to the parent star to be covered by its light. More importantly, it requires a powerful enough telescope equipped with a coronagraph.

Exoplanet HIP 65426b as seen by the NIRCam and MIRI instruments on JWST

Image credit: NASA

4. Gravitational Microlensing is a method of detecting exoplanets by measuring changes in stellar brightness.

Gravitational lensing is an optical effect predicted by Einstein's general theory of relativity. Since space-time is distorted near massive celestial bodies, light will bend when passing near massive celestial bodies. If there is a massive celestial body in the straight line from the observer to the light source, the observer will see one or more images formed by the bending of light. This phenomenon is called gravitational lensing. If the mass of the foreground celestial body is small, the deflection of light is also small, and the multiple images produced at this time will be difficult to distinguish. The visual effect is that the brightness of the background star is significantly enhanced. When the foreground star and the exoplanet happen to pass through a background star, the brightness of the background star will increase, and a peak will be generated on the light curve. The mass of the exoplanet is smaller than that of the foreground star, and the peak produced will also be smaller, but it can still be observed that a smaller peak is superimposed on a certain position of the original light curve. We can determine the existence of an exoplanet by the secondary peak produced by the light curve.

Principle of Microlensing

In addition to the above-mentioned mainstream methods, there are also many other methods such as astrometry, transit time variation method, brightness modulation method, etc. However, these methods have only been discovered sporadically and are rare in number.

Radius-period distribution and mass-period distribution of discovered exoplanets

Statistics of discovered exoplanets

As of September 2, 2024, there are a total of 7,323 confirmed exoplanets, of which 4,456 were discovered by the transit method. In addition, 1,273 were discovered by the radial velocity method, 1,051 by direct imaging, and 308 by microlensing. (Data source: Encyclopedia of Extrasolar Planets http://exoplanet.eu/)

The significance and prospects of exoplanets

The study of exoplanets has important scientific value. It not only provides valuable data for us to understand the formation mechanism of planetary systems, but also provides a comparison object for the solar system. The discovery of Earth-like planets, especially those in the habitable zone of stars, greatly increases our chances of discovering extraterrestrial life.

As technology improves, especially with the advent of new-generation telescopes like the James Webb Space Telescope (JWST), we will be able to study the atmospheres of exoplanets more deeply and perhaps even directly observe signs of life.

In addition, the discovery and study of exoplanets has greatly increased the public's curiosity about the universe and promoted the development of scientific education. The exploration of exoplanets is an exciting field that not only promotes the development of science and technology, but also inspires people to think deeply about the possibility of life. With the continuous advancement of observation methods, we look forward to revealing more mysteries about exoplanets in the future and ultimately answering the age-old question: Are we alone in the vast universe?