Analysis of Extra-Planets Searching and Detection Approaches:

Radial Velocity, Transition and Gravitational Microlensing

Jiayi Zhou

School of Physics, University of Bristol, Bristol, U.K.

Keywords: Exoplanets Detection, Radial Velocity, Transit, Gravitational Microlensing.

Abstract: The search for exoplanets has become one of the most dynamic and rapidly advancing fields in modern

astronomy. Understanding these distant planets expands our knowledge of planetary systems and provides

crucial insights into the formation and evolution of the solar system. This study compares three major

exoplanet detection techniques, i.e., radial velocity, transit photometry, and gravitational microlensing. By

examining their detection principles, strengths, and limitations, the study provides a theoretical foundation

for selecting appropriate methods under different observational conditions. Radial velocity is particularly

effective for detecting massive planets around nearby stars and measuring planetary masses; transit

photometry is well-suited for identifying smaller, close-in planets; and gravitational microlensing is uniquely

capable of revealing distant and low-mass planets, including those beyond the snow line. Given the

complementary nature of these techniques, the paper highlights the growing importance of a multi-method

approach to improve detection accuracy and broaden the scope of exoplanet discovery. This comparative

framework offers practical guidance for researchers in choosing optimal detection strategies based on specific

scientific goals and target characteristics.

1 INTRODUCTION

Humans have started to investigate planets in very old

history in many cultures. In ancient societies,

people’s findings of planets are expressed by myths,

large-scale devices, calendars, etc. For example, the

Popol Vuh of the ancient Maya counts the creation of

the “sky-earth”, Stonehenge in England (~3000 BCE)

was designed according to celestial events. In 1543,

Copernicus first proposed that planets move in orbits

around the Sun. Then Galileo’s planetary observation

and Kepler’s laws of planetary motion were

discovered, which was incorporated into a new

physical explanation of the planets. In 1687, Newton

succeeded in describing the universal force of gravity

mathematically presenting that the movements of

planets are the results of a gravitational relationship

(Shindell, 2023). The research before the 20th century

focused on the motion laws of planets. With modern

techniques’ developments, the detection of planets’

meanings varies. For example, discovering and

exploring habitable planets and searching for possible

extraterrestrial lives, like the CHES’s project (Ji, et al.,

https://orcid.org/0009-0004-8584-6263

2022), detecting extrasolar planets to understand the

origin of the solar system, like the Orion project

(Black, 1980), or just for the reason of deepening the

understanding of planetary systems, like the Kepler

mission (Borucki, et al., 2007).

Despite the breakthrough in astronomy, the

detection of planets would promote the development

in related areas, including geology, planetary science,

astrochemistry, astrobiology, and atmospheric

science (Zhou, et al., 2024). According to the

Encyclopaedia of Explanatory System websites, 7423

planets have already been discovered in different

methods by March 16, 2025. As the amounts of

discovered planets increased, it provided researchers

with a deeper understanding of the diversity of the

cosmos. Researchers classified different planet types

due to their physical properties and orbital (Muresan,

2025). Despite the remarkable achievements that

researchers have made, there are still several critical

questions unsolved. More reliable and efficient

detection is needed, to break the challenges and

answer the questions.

158

Zhou, J.

Analysis of Extra-Planets Searching and Detection Approaches: Radial Velocity, Transition and Gravitational Microlensing.

DOI: 10.5220/0013815800004708

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 2nd International Conference on Innovations in Applied Mathematics, Physics, and Astronomy (IAMPA 2025), pages 158-164

ISBN: 978-989-758-774-0

Although different methods of planetary detection

have been proposed and practiced, such as radial

velocity, transit, microlensing, and direct imaging,

the different techniques have significant differences

in many aspects, which makes it difficult for

researchers to select a suitable method. Moreover,

with fast development in the planetary detection field,

more findings and technologies are presented, and a

comprehensive review is needed to organize the

information on different methods of planetary

detection.

The research aims to analyse the details of 3 main

methods of planetary, transition, radial velocity, and

gravitational microlensing, explain the advantages,

limitations, and application scenarios of each method

by conducting existing literature, helping researchers

choose the proper method when given different

conditions.

2 DESCRIPTIONS OF PLANET

SEARCHING

Planet searching covers a great range of physical

quantities, including quantities about orbital, mass,

atmosphere, inner structure, fluxes, and planetary

surfaces. For extrasolar planets, researchers based

mostly on radial velocity measurements, transition

measurements, imaging and microlensing to gain

planetary information. In early planetary detection

discovery before 2014, it was the most common to

use the measurements of radial velocity. However,

with equipment’s promotions, researchers can detect

the planets with longer distances by using transition

information. Until January 21st, 2025, 5819

exoplanets were confirmed, with about 74% detected

by using the transition method, 19% by measuring the

radial velocity, and 4% by using gravitational

microlensing. Despite the other methods were not as

common as the three methods above, they still helped

with about 1% of planets were detected for direct

imaging, 0.6% for transition time variation, and

0.05% for astrometry (Muresan, 2025).

3 RADIAL VELOCITY

In 1995, Machel Mayor and Didier Queloz

discovered the first exoplanet orbiting a sun-like star

using the radial velocity method and named it 51

Pegasi b. Although the information provided by radial

velocity signals is limited, RVs are very successful at

detecting exoplanets. The radial velocity is based on

the Doppler shift effect on the spectrum of a star when

the planet and its host star are rotating around the

same mass centre. This orbit makes the star appear to

wobble in the sky. Although this change in position

can be detected using the astrometry method, it can

be more sensitive for researchers to determine the

shift in wavelength of the starlight, due to the periodic

Doppler effect when the star rotates around the mass

centre of the system. When the star approaches the

receiver, the frequency of light will increase, then its

spectrum will move towards the blue side, and the

shift is therefore called the blue shift. Similarly, the

spectrums move towards the red side for being away

from the observer as shown in Fig. 1 (Muresan, 2025).

By measuring the frequency change in a star’s

spectrum, researchers can determine how fast the star

moves toward or away from the observers due to the

gravitational interaction between the star and orbital

planets.

Figure 1: A sketch of the Doppler effects.

Analysis of Extra-Planets Searching and Detection Approaches: Radial Velocity, Transition and Gravitational Microlensing

159

Because of, the star’s orbit, the distance from the

system’s mass center to the star will increase as the

ratio of the mass of the star and planet approaches 1,

the shift is more obvious. The method thus is more

sensitive in the detection of large-mass planets (Brogi,

et al., 2013; Lovis & Fischer, 2010). That’s also the

reason why the first exoplanet is 51 Pegasi b because

it is a gas giant and typed in hot Jupiter with a

minimum mass of about 0.47 times that of Jupiter.

However, it can also be used for terrestrial planets

although it is still very challenging. For example,

Nieto & Díaz succeeded in training a convolutional

neural network algorithm to detect planets with a

mass of about 4 times that of the Earth, even if the

radial velocity is as low as 5 meters per second.

However, the radial velocity method can only

determine the minimum possible value of the planet’s

mass, since the orbital inclination is unknown. The

measurements only cover the motion of the star along

the line of sight, which means the radial velocity

signal can be weakened if the angle between the sight

line and orbit is large. Another limitation is that the

interference from stellar activities can also affect the

spectrum of starlight. Stellar activities like sunspots

and flares can cause spectral line shifts, besides some

of these activities are periodic as well. So, it will be

hard to avoid this kind of interference. With the

improvement of techniques, researchers are now

trying to mitigate this issue (Zhao, et al., 2024).

Besides, the method may be limited to determining

the shift of planets with high speed because the fast

movement broadens their spectral line and makes the

determination not precise enough. In summary, the

radial velocity method is more likely to be used in the

detection of inactive, cool, and slowly rotated stars

with typical spectrum types of F, G, and K.

The instruments used in the radial velocity

method are highly specialized now, which can

achieve the high precision needed to detect the tiny

Doppler shifts caused by orbiting planets.

Spectrometers are the most common instrument used

in radial velocity. It detects shifts in the spectrum

from stars. There are two common types of

spectrometers used in radial velocity: echelle

spectrometers like HIRES at Keck Observatory and

fiber-fed spectrographs like FEROS at the European

Southern Observatory (ESO). Spectrometers are

usually equipped on large telescopes that collect and

focus the light from distant stars. The size directly

impacts the amount of light gathered and the

resolutions of observations. To provide a precise

measurement, it is also important to keep the optical

systems stable. The equipment is usually placed in a

temperature-controlled environment to minimize the

errors in the spectrum measurements. As technology

advances, future instruments aim to push the

precision further to reliably detect Earth-like planets

around Sun-like stars.

Although the number of exoplanets found by

radial velocity is not as much as it is in the history of

recent years, the contribution of the method can still

be important. Besides redial velocity has achieved an

extreme precision of 10 𝑐𝑚 𝑠



, which means the

method can be used for detecting exoplanets with

fewer limitations. Besides, recent research is intense

to combine measurements of other methods to get

more detailed and accurate information (Hara & Ford,

2023). Future detection using radial velocity can

deserve the researcher’s expectations.

One specific example is the detection of Barnard's

Star b, using a high-precision spectrometer

ESPRESSO, detected by Jonay González

Hernández’s team in 2024. Barnard’s Star B is a red

drawl with a distance of 6 light-years from Earth.

Because its minimum mass is relatively low, about

0.3 times that of Earth, it is hard to detect without a

high-precision instrument. The detection of Barnard’s

Star B may demonstrate that if the precision gets

higher, it is possible to use the radial velocity method

to search for more exoplanets.

4 TRANSITS

The emergence of the transit method is a significant

event in exploring planets. Since, 1631, Pierre

Gassendi first observed the transition of Mercury

across the Sun (Fisher, 2005), the transit method was

used to study the subjects in the solar system. In 1999,

astronomies succeeded in determining the existence

of exoplanet HD 209458b using the transit method

(Snellen, et al., 2010). It was proved to be valid in the

detection of exoplanets. The transit method has

helped researchers detect more than 4000 exoplanets

and become the most crucial technique for

discovering the universe. As technology progresses,

the transit method will still play an irreplaceable role

in exploring exoplanets.

When a planet transits the star, a small part of

starlight cannot pass through the planet, and, thus, the

measurement of its magnetoelectronic wave will

experience a significant drop in its light curves, which

shows the star’s fluxes over time. The light curve

shows a “U” shaped dip when the transit occurs. As

the planet starts to enter the disk of the star, the light

curve begins to fall until the whole body of the planet

enters the disk. During the time that the whole planet

is in the star’s disk as the example of HD 209458 b

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shown in Fig. 2 (Haswell, 2010), the light curve

would stay at a relatively low value before a part of

the planet moves off the disk, which is directly related

to the size of the planet and the star. Then, the light

curve would rise gradually back, until the planet is

totally off the disk (Deeg & Alonso, 2018).

Figure 2: The impression of the transit of HD 209458 b

across its star (Haswell, 2010).

The transit method provides valuable information

about the planet, such as its size, and orbital period.

Since the brightness dip in light curves’ relation to the

sizes of planets, researchers can determine planets’

sizes relative to the stars by analysing the depth of the

transit depth. If the planet’s orbit is stable, the period

of the planet can be determined by measuring the time

between each transit event (Nesvorný & Morbidelli,

2008). In some cases, due to some compositions in

the planets’ atmospheres can absorb

magnetoelectronic waves of wavelengths, by

analysing the spectral changes during the transition

researchers can study the temperature, compositions,

and structure of their atmosphere (Sing, 2018).

The transit method can be considered the most

successful method to detect exoplanets, for it allows

the discovery of many exoplanets with a great

quantity of data, especially when it is combined with

the radial velocity method. It is highly efficient in

terms of time and effort because the monitoring of a

star’s fluxes over time can be done continuously

without interruption. Space telescopes like Kepler

and TESS, it can observe hundreds of thousands of

stars’ fluxes concurrently, which means the method

can be used for large-scale detection. The precision of

Kepler and TESS allows researchers to detect a

minimal variation in light curves. Then, the method is

good for detecting Earth-like or even smaller

exoplanets, which other methods might miss (Winn,

2024; Bruno & Deleuil, 2021). The transit depends on

stable, wide-field, high-cadence detectors, most

effectively applied in space to avoid atmospheric

effects. Missions like Kepler and TESS have

significantly expanded the known population of

exoplanets. The refinement of both instrumental

sensitivity and observational strategies will ensure

that transit-based surveys remain at the forefront of

exoplanet detection and characterization. These

instruments, especially when paired with radial

velocity and atmospheric spectroscopy, are central to

the future of exoplanet science.

While the transit method is highly successful in

detecting exoplanets, several limitations and

challenges exist. One of the limitations is that the

planet’s orbit must be aligned in the plane of its star

and the Earth. That means one can only observe a

fraction of exoplanets aligned along our line of sight.

Besides, to confirm the planets’ existence,

researchers need to observe several times of

transitions, which might need several years of

observation if the orbital period of the planet is

relatively long. The transit methods can also be

affected by a variety of stellar activities including star

spots, stellar flares, and stellar pulses causing fake

positives.

TOI-2180 b1 is a gas giant exoplanet that was

discovered and confirmed to exist in 2022 using the

transit method with TESS. It has a period of 260 days

with a moderate orbital eccentricity of 0.37. The

planetary existence was then validated by the radial

velocity method (Dalba, et al., 2022).

5 GRAVITATIONAL

MICROLENSING

The gravitational microlensing is an astrophysical

phenomenon that occurs when the light from a distant

star travel through the gravitational fields of a

massive object, such as stars and black holes. The

gravitational field with great strength would act as a

lens and magnify the magnetoelectronic waves. This

effect is a consequence of Einstein's theory of general

relativity, which predicts that light will bend around

a massive object as depicted in Fig. 3 (Zakharov &

Sazhin, 1998). In the situation where the lensed star

is perfectly aligned with the massive object, the ring

image would become apparent (Zakharov & Sazhin,

1998). The effect can also be observed by analysing

the light curve. When the light travels through the

lens of the gravitational field, the light curve would

experience a sharp increase in fluxes. The

magnification depends on the alignment of the light

source and lens and the peak of fluxes is generated

now when the lens is closest to the line between the

earth (observer) and the light source. In the field of

Analysis of Extra-Planets Searching and Detection Approaches: Radial Velocity, Transition and Gravitational Microlensing

161

exoplanet detection, massive exoplanets would act as

the microlensing. The planet’s effect on the

microlensing light curve would create a characteristic

increase in the fluxes, and the presence of the planets

can be determined in this method, in the case that the

orbit of the planet is nearly perpendicular to the Earth,

which makes the radial velocity and transit method

hard to practice.

Figure 3: The massive object acts as lens L located between

source S and observer O produces two images S1 and S2 of

the background source (Zakharov & Sazhin, 1998).

The gravitational method has helped to find over

400 exoplanets. However, the gravitational

microlensing method comes with several significant

limitations, particularly because of its rarity. Since the

microlensing events require a very precise alignment

between a ground source, the lensing objects, and the

observer. Since the alignments happen randomly, it is

hard for researchers to predict the microlensing

events long in advance. Thus, to catch the potential

events, researchers must monitor millions of stars

continuously. Unlike the radial velocity and transit

method, gravitational microlensing events are mostly

single-time measurements. This makes it almost

impossible to refine the measurements of planets in

followed observations, once the lens and source have

moved out of alignment (Wambsganss, 1998;

(Rektsini & Batista, 2024).

Observing gravitational microlensing events

requires specialized instrumentation capable of

continuously monitoring dense star fields with high

photometric precision. These telescopes provide the

high cadence and continuous observation necessary

to capture the short-lived light curve changes caused

by microlensing planets. One example is the project

Korea Microlensing Telescope Network（KMTNet),

which operates telescopes in Chile, South Africa, and

Australia. The KMTNet telescope is equipped with

high-sensitivity CCD cameras to play a central role in

detecting the possible gravitational microlensing

method in the universe (Kim, et al., 2016).

MOA-2022-BLG-249Lb, a nearby super-Earth

exoplanet 2000 par seconds away from the Earth, was

detected via gravitational microlensing in 2022. The

planetary mass was determined to be about 4.83 times

that of the Earth. It orbits a host star with a mass of

0.18 times that of the sun. The gravitational

microlensing event happened with a brightness

deviation of 0.2 magnitudes during about 1 day (Han,

et al., 2023).

6 COMPARISON, LIMITATIONS,

AND PROSPECTS

Despite the three methods mentioned above being

widely and commonly used in the exploration of

exoplanets, each method has its strengths, limitations,

and ideal use conditions. The radial velocity method

is particularly effective for detecting massive planets

in orbits with a small radius, but it is limited to being

sensitive in the search for small planets, particularly

for those far from their stars or in multi-planet

systems. Another limitation is that the radial velocity

method needs highly stable spectrums to operate,

which means without stability the spectral shifts

could be mistaken for stellar activities.

Transits are highly effective for detecting small

planets, even those Earth-sized, but the method

requires near-perfect alignment between the planetary

orbit and our line of sight. For this reason, many

planets may be undetected because of their angles. A

critical aspect of the transit method is the need for

high-precision photometric measurements and

continuous monitoring of millions of stars over

extended periods.

Gravitational microlensing is a less common but

powerful method to detect farther exoplanets.

However, microlensing events do not repeat, making

confirmation and further studies challenging. A single

event caused by the presence of a planet orbiting the

lens star often lasts only a few hours. Capturing such

short-lived deviations requires observations every

few minutes which strongly demands telescope

scheduling, data processing, and instrument readiness.

The paper primarily focuses on three widely used

exoplanet detection methods: radial velocity, transit,

and gravitational microlensing. However, the paper

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does not discuss other detection methods such as

direct imaging and astrometry. Despite their

effectiveness in specific conditions, they do not

belong to the main scope of the paper. Future research

is expected to combine and compare the results from

multiple observational methods. By integrating

different methods, researchers can gain a more

comprehensive understanding of exoplanetary

discovery. This multi-method synergy not only

improves the confirmation of planetary candidates

but also expands the diversity of planetary types and

environments one can detect.

7 CONCLUSIONS

To sum up, this research has examined and compared

three of the most prominent exoplanet detection

techniques: radial velocity, transit photometry, and

gravitational microlensing, providing researchers

theoretical basis for choosing proper methods in

different conditions of exoplanetary detection. Each

method offers distinct advantages. Radial velocity is

best suited for detecting massive planets around

nearby stars and confirming planetary masses; transit

excels at identifying planets with lower masses and

gravitational microlensing is uniquely effective for

detecting distant planets. Given these complementary

strengths and weaknesses, it is increasingly clear that

the most effective strategy for exoplanet detection

and characterization lies in the integration of multiple

methods. By illustrating and comparing radial

velocity, transit, and gravitational microlensing, the

paper helps to clarify the roles played by these three

most widely used methods and offers guidance to

researchers in selecting optimal detection strategies

for different scientific contexts.

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