Probing Exoplanets Based on Transit, Radial Velocity and Direct

Imaging

Liaoyuan Ma

Jinan Thomas School, Jinan, China

Keywords: Exoplanet Detection, Radial Velocity, Transit, Direct Imaging.

Abstract: As a matter of fact, human beings have evolved step by step from primitive humans to nowadays, never giving

up on exploring the universe. Since Soviet cosmonaut Yuri Gagarin took off on the Vostok 1 spacecraft,

human began to explore the universe frequently. So far, preliminary progress has been made in the study of

exoplanets. In reality, the exploration of exoplanets will have a profound impact on the development of human

civilization, and may lead to the discovery of a second Earth suitable for human survival. With this in mind,

this article will introduce some commonly used methods for discovering exoplanets, and provide a detailed

introduction to the three most common methods and their principles, instruments, and related results.

According to the analysis, this study summarizes the present problems and difficulties that limiting exoplanet

detecting and looks ahead to the promising developments of planetary exploration in the future. These results

provide a guideline in further discoveries of exoplanet.

1 INTRODUCTION

Earth is currently the only planet determined by

humans to have life. Scientists have long hoped to

discover other exoplanets outside the solar system

that are fit for the existence of life. Ever since the first

exoplanet has been discovered to be orbiting a sun-

like star in 1992 (Mayor and Queloz, 1995). The

exoplanet has an enormous breakthrough in the last

30 years. The search for exoplanets has become one

of the most active topics in astronomy and

astrophysics (Mamajek and Stapelfeld, 2024). Some

typical important progress is summarized in Fig. 1

(Zhou et al., 2024).

The systematic observation and theoretical

characterization of exoplanets have some scientific

significant. First, it will greatly deepen people's

comprehending of the past as well as future of Earth

and the solar system, which is relevant to the future

development of mankind. Secondly, the observations

of exoplanets have provided critical data to test

theories of the growth and evolution of the solar

system, which has greatly improved the

understanding of the derivations of the Earth and life.

Thirdly, it will also have the potential to directly

answer the major scientific question of whether

humans are alone in the universe, and is a prerequisite

for the search for habitable planets and even the

establishment of extraterrestrial homes condition

(Zhou et al., 2024).

Ever since the initial exoplanet was found in 1995,

research into exoplanets has involved participation

from every country. Both the second Kepler mission

and NASA's Kepler mission proposals were seen as

enormous successes (Borucki et al., 2010). The

discovery of the first confirmed rocky exoplanet

received a lot of media attention. Using this kind of

process, 548 exoplanets emerged between 2009 and

2018. The most successful detection experiment to

date is TESS, its successor and a representative

instrument of the transit method. The world’s

optional telescope now, the Very Large Telescope of

the European Southern Observatory (VLTESO) is

used for planet discovery. Beyond technical

constraints, its equipped ESPRESSO achieves

10cm/s precision and significantly improves

detection accuracy (Pepe et al., 2014). The JWST

Space Telescope is scheduled to launch in 2021

(JWST group, 2023). As of right now, JWST has

accurately measured the composition of the

atmospheres of some exoplanets (including H

O, CO

and CO, CH

), and has also discovered several

wandering binary planetary systems. This planet

system has majority of planets found beyond the solar

system to date: 5,514 exoplanets had been found as of

Ma, L.

Probing Exoplanets Based on Transit, Radial Velocity and Direct Imaging.

DOI: 10.5220/0013043200004601

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

In Proceedings of the 1st International Conference on Innovations in Applied Mathematics, Physics and Astronomy (IAMPA 2024), pages 247-252

ISBN: 978-989-758-722-1

247

September 10, 2023, primarily due to human

competence in more than ten exoplanet methods of

detection (Yock and Muraki, 2018).

This article aims to provide an overview of the

most widely used exoplanet methods for detection

currently in use, as well as display their principles and

related instruments. The Section 2 will introduce the

mainstream exoplanet detection methods and

detectors. In Section 3, the study will concentrate on

the principle of detection method of transit, related

instrument and concrete detection results in detail.

Section 4 will include specific for the particulars of

the radial velocity, the relevant instrument, and its

discovery outcomes. The measuring principle,

instrument application, and significant advancements

in direct imaging will be covered detailed in the fifth

section. In Section 6, the current obstacles to

exoplanet detection progress will be systematically

evaluated along with the opportunities and challenges

that lie ahead. The article will be summarized in

Section 7 together with an optimistic outlook for

exoplanet discoveries.

Figure 1: Important progresses and space telescopes in exploration of exoplanets (Zhou et al., 2024).

2 DESCRIPTION OF PLANET

As analysis technology advances and equipment

accuracy improves, humans have perfected a variety

of planet detection techniques. These ways can be

split into 2 main categories: indirect detection

methods and methods of direct imaging. Indirect

detection approaches comprise pulsar timing,

astrometry, gravitational microlensing, arch astrolabe,

radial velocity, transit method, orbital brightness

adjustment method, planetary disk motion and remote

sensing. This article will focus on three commonly

used methods: radial velocity, transit, and direct

imaging but not in the section. In this section, the

study will introduce some common methods for

discovering exoplanet and pertinent instrument

briefly.

Astrometry is the earliest approach of finding for

exoplanets. This method is to precisely locate the

location of a star in the firmament and watch how that

location alters over time. In case the star has a planet,

the planet's gravity will lead to a star to move within

a minuscule circular orbital. In this case, the stars and

planets rotate round their widespread center of mass

(the binary problem). Since the mass of a star is much

enormous than that of a planet, its orbit is much

marginal than that of a planet (Sahlmann and Fekel,

2013).

By observing the effects of exoplanets on a host

star during their revolution, the timing method finds

evidence of their presence. The pulsar's radiation

emissions are very constant and fixed because of it

revolve, because the revolve of a pulsar is steady in

natural situation, so time anomaly detected on the

radio wave radiation of the pulse is capable to

calculate the motion of the pulsar. Owing to normal

stars, pulsars move in small orbitals if they have

planets. It is possible to compute and infer the orbital

parameters from the pulsar's pulse time. The first

exoplanet was discovered around the pulsar PSR

1257+12 (Wolszczan and Frail, 1992). The major

disadvantage of the pulsar timing method is which

pulsars are relatively unusual, so it is not possible to

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use this method to discover planets in large quantities.

Meanwhile, as it known to all, life cannot exist on

planets orbiting pulsars because of the very intense

high-energy radiation.

Gravitational microlensing is a unique technique

for discovering exoplanets as it is independent of the

host star's brightness or the planetary system's orbita

l motion. When a star's gravitational field acts like a

lens, amplifying the light of a distant background star,

the behavior of a gravitational microlensing is

generated. This influence is only produced when the

two stars are almost perfectly aligned. Only when the

two stars move towards each other and the Earth are

exactly in opposite positions, so the lensing event is

short-lived and can only last for a few days or weeks.

Thousands of such incidents have been located over

the past decade. In case the star in the foreground has

planets, then the lensing effect contributed by the

planet's gravitational field can also be detected. Since

this requires very precise alignment to detect the

microgravitational lensing effect of the planets, it is

vitally need to monitor a very large number of stars to

have a reasonable chance of observing this

phenomenon. The most likely outcome of this

approach is to observe the stars that are situated

between Earth and the Milky Way's core, which can

provide a large number of background star. until the

first discovery of microlensing planets in 2003, more

than 200 planets have been identified by means of

gravitational microlensing (Rektsini and Batistam

2024). This is the barely approach permissible of

locating Earth-sized planets round ordinary main-

sequence stars.

The Doppler brightening effect, in which the seen

frequency deviates from the real frequency and the

host star's brightness varies depending on the planet's

radial velocity, can be brought on by the rotation of

an exoplanet around the host star. An iconic example

of the method is the NASA James Webb Space

Telescope (JWST). It is called orbital brightness

adjustment.

The planetary disk motion uses submillimeter

light wave arrays to measure variations in the

protoplanetary disk's substructure to infer the

existence and development of exoplanets based on

current hypotheses and observational data. One direct

use of this technique is ALMA detection. Even

though this technology has only been used to confirm

one exoplanet thus far, the method's potential

applications are growing and its future seems bright.

Remote sensing is a popular technique for planet dis

covery. Using equipment on the surface of ground, on

satellites, or in the air, remote sensing methods

collects data on planets and other celestial bodies.

Seager and Deming summarized recent developments

in the study of planetary atmospheres in their review

paper. These advancements involved observations

and models to disclose the properties of planetary

atmospheres. For example, spectral features in

planets' atmospheres can be observed, which is

applicable to disclose the properties of planetary

atmospheres through modeling and measurements

(Seager and Deming, 2010).

3 PROBING BASED ON TRANSIT

The most practical and widely used method is the

transit method; 4,115 exoplanets have been found

using it. This is because detection projects like Kepler,

K2, and TESS have developed successfully. The

transit method's fundamental principal is to exploit

the eclipse event, which occurs when a planet

revolves around its primary star, and use satellites to

track the main star's brightness drop to detect the

presence of exoplanets. When a planet moves ahead

of the star or blocks it, it is referred to by academics

as a secondary eclipse. Primary eclipses occur when

a planet passes in front of the star. The term

"occultation depth" also refers to the primary star's

degree of brightness drop. Because the brightness of

the primary star in a secondary eclipse diminishes less,

the existing detection methods only track the primary

eclipse. The strengths of the light curve can be used

for estimating a planet's size via the transit approach.

Integrated with radial velocity, that can measure the

mass of the planet, so the density of the planet can be

determined, and then more information can be known

about the physical structure of the planet. Nine of the

known exoplanets have known their best

characteristics through these two methods. Transit

can also research the atmospheres of exoplanets. As

the planet crosses in front of the star, the star's light

passes through the planet's upper atmosphere.

Researching the stellar spectrum of stars can identify

the elements present in the atmosphere of the

traveling star. It is also possible to measure the

polarization of light as it travels through the

atmosphere of the planet or is reflected by it, and to

detect the composition of the planet's atmosphere and

the planet's material.

Planets that Earth-like and are located in or

surrounding to the Milky Way's habitable region,

ranging in size from 0.5-2 times that of Earth. Kepler

had discovered 2,778 exoplanets as of May 2013. To

carry out the Kepler mission, K2, Kepler's second

mission, was formally launched in May 2014

Probing Exoplanets Based on Transit, Radial Velocity and Direct Imaging

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(Barclay, et al. 2018). Some typical results are shown

in Fig. 2 (Yang, 2024).

Figure 2: A sample for system that TESS discovered

two or more transiting planet(right) a sample of M

dwarf system that TESS found a single transiting

planet (left) (Yang, 2024).

4 PROBING BASED ON RADIAL

VELOCITY

Radial velocity, which is similar to astrometry,

determines the velocity at which a star move closer or

farther from the Earth by taking into account the fact

that it moves in a brief circular orbit due to planet

gravity. According to the Doppler effect, the radial

velocity of a star can be determined by the motion of

the spectral lines of the star. Once a star is in a basic

two-body system, its orbital around the planet's mass

center will be elliptical because of the planet's gravity,

which causes some oscillation in the star. wave source

and the observer change the wavelength of an object's

radiation. When a star travels in a straight-line

direction toward Earth, its lines will shift blue, and

when it moves in the other direction, the spectral lines

will shift red. The wave source's velocity as it moves

toward the observation can be computed rely on the

wave's degrees of red (or blue) shift. Using a

spectrum analyzer mounted on a telescope, one can

detect changes in spectral lines, or variations in a

star's radius of variation or position over time, in

order to identify the presence of exoplanets. The

radial velocity approach, which yielded the initial

exoplanet discovery, is very effective at finding

planets in nearby and far-off star systems. One of the

most significant strengths is that it permits the

eccentricity in the planet's orbital to be calculated

immediately. Although the radial velocity signal is

not being influence of distance, high signal-to-noise-

ratio spectra are necessary for high degree accuracy.

A sketch is shown in Fig. 3.

Figure 3: Diagram of Doppler effect in the 2-body

systems

(Zhou et al., 2024).

The James Webb Space Telescope (JWST),

whose is launched in 2019, have a significant positive

impact on exoplanet-hunting investigates by means of

radial velocity. When this mission is in operation, it

will use its sophisticated set of infrared instruments to

measure the Doppler radii of stars in order to search

for potential exoplanets.

5 PROBING BASED ON DIRECT

IMAGING

The direct detection method can more precisely and

thoroughly characterize the features of exoplanets,

although it does require higher instrument accuracy,

such as Gemini Planet Imager and VLT. Telescopes

can only observe exoplanets with direct images under

special circumstances. Specifically, direct images are

only relatively easy to obtain when the planet is

massive (usually much massive than Jupiter) and far

enough away from the parent star to radiate a large

amount of infrared light. Less false positives are one

of the most evident benefits of using Direct Imaging.

Although there aren't many possibilities to use this

method, when direct detections can be made, it can

give scientists crucial comprehensions on the planet.

Astronomers can discover significant details about a

planet's component, for example, by analyzing the

wavelengths reflected by the atmosphere. The

information is corresponding to exoplanet’s

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characters and judge if it is potential for survival of

human.

Figure 4: The CCD frame of 2M1207b [14].

On December 25, 2021, the James Webb Space

Telescope (JWST) was launched into space, seen

from Fig. 4 (Kalirai, 2018). NASA released the first

direct image of the HIP 65426b obtained by JWST on

September 1, 2022. Three dust rings were detected

around this star, two of which were found for the first

time. Water vapor was discovered by James Webb

Space Telescope on July 24, 2023, near the region

where rocky planets are formed. JWST detected

carbon dioxide and methane in the atmosphere of K2-

18b on September 11, 2013. With this assistance in

the future, astronomers will still be able to study and

understand the how the cosmos evolve, the

formations and components of planet system, and the

prospect of life in this system. In 1983, astronomer

Edwin Powell Hubble received official recognition as

the namesake of the Hubble Space Telescope (HST).

After more than 40 years of planning, it was launched

on April 24, 1990, from the Kennedy Space Center

(KSC) in the America. A telescope in orbit surround

the Earth. It is not affected by atmospheric turbulence

because it is situated above Earth's atmosphere. It can

detect ultraviolet light absorbed by the ozone layer

and produces spectra and pictures that are remarkably

steady and repeatable. The Hubble Space Telescope

(HST) has altered the understanding of the universe

and advanced the knowledge of it since its launch

through the observation of data and photographs.

Astronomers reported in 2001 that the STIS had

effectively analyzed the exoplanet HD's atmosphere

component (Debes et al., 2019). The exact mass of the

Gliese 876b was found in 2002 with the aid of the

Hubble Space Telescope. In 2008, Hubble detected

carbon dioxide and methane on 2 exoplanets. In 2008,

it took the first photo of an exoplanet. Hubble identify

7TRAPPIST-1 system elements in 2017. Liquid

water was discovered on the outside of an exoplanet

the size of Earth's atmosphere. In 2018, a potential

exoplanet was discovered. The first detection of water

vapor in the atmosphere of the exoplanet K2-18b

occurred in 2019.

6 LIMITATIONS AND

PROSPECTS

The exploration of exoplanets is a domain of great

scientific value as it may help people find new homes

in the future. However, currently, the major method

of finding exoplanets are radial velocity, direct

imaging, and transit methods; however, each of these

methods has inherent drawbacks. Transit, for

example, is unable to capture photos or gather

spectral data on the planet itself. Only brief variations

in light brought about by a planet moving in front of

its parent stars can be detected by it. Extremely high

telescope resolution and exact image processing are

required for direct imaging of planets to divide them

from the. Years of constant detections are needed for

radial velocity to precisely calculate a planet's mass

and orbit. Furthermore, powerful telescopes and

devices are needed to find exoplanets. For example,

to produce high-resolution and high-sensitivity

photographs in darkness, direct imaging necessitates

exceptionally high-performance telescopes and

specialized equipment. With radial velocity,

precision optometric devices and powerful computers

are needed to estimate the planet masses and orbital

characteristics. These reasons all lead us to only

discover exoplanets that are relatively close to Earth,

but unable to discover exoplanets that are farther

away from us.

The study of exoplanets will face increasing

possibilities as science and technology advance. More

advanced instruments and methods of detection will

be used in the future. The knowledge of the

characteristics and evolution of exoplanets will

increase owing to these novel instruments and

methods for observation. Analyze the temperature,

composition of the atmosphere, and alterations in

climate of exoplanets to acquire a better

comprehending of their ecology and evolution.

Understanding the start and growth of life in else

planet systems will be aided by this. Examine

exoplanets' physical features, like mass, size,

atmosphere, and surface structure, by spectroscopic

Probing Exoplanets Based on Transit, Radial Velocity and Direct Imaging

251

or direct imaging methods. This will offer insights on

the genesis and development of planets.

7 CONCLUSIONS

Overall, the space technology has advanced

significantly because of scientific and technological

advancements, it will have a crucial impact on the

future development of human civilization. This study

provides a comprehensive analysis and summary of

the radial velocity, transit, and direct imaging

techniques used in planetary exploration, along with

information on their possible uses and limitations.

Additionally, it describes a few additional widely

used methods in finding exoplanets, include

gravitational microlensing and astrometry. Even if

there are still many challenges and barriers for

humanity to solve, methods and advances in

technology are constantly developing. Because it will

increase the chances of surviving, improve the

knowledge of the cosmos and the Earth, and have a

significant impact on and alter the future, interstellar

exploration holds immense importance for humanity.

This work will offer some essential references for

planetary exploration and research in the future.

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