21.3 Evidence That Planets Form around Other Stars
3 min read•june 12, 2024
Planet formation is a cosmic dance of dust and gravity. Tiny particles in protoplanetary disks collide and grow, eventually forming . These building blocks then merge to create rocky planets close to stars and gas giants farther out.
Observing young star disks gives us clues about this process. We can see evidence of planet formation in disk structures like gaps and spiral arms. These observations, along with exoplanet discoveries, help us understand how planets form around other stars.
Planet Formation around Other Stars
Evolution of protostellar dust
forms around a newly born star from the collapsing molecular cloud, composed of gas and dust particles
Dust particles collide and stick together due to electrostatic forces, growing from micron-sized to centimeter-sized particles
form as centimeter-sized particles further collide and grow into kilometer-sized objects, serving as the building blocks of planets
Terrestrial planets form in the inner disk as planetesimals collide and accrete to create rocky planets, with high temperatures close to the star preventing the accumulation of ice and gas (Mercury, Venus, Earth, Mars)
Gas giants form in the outer disk where lower temperatures allow ice particles to add to the mass of planetesimals, with massive planetesimals (around 10 Earth masses) gravitationally attracting and accreting gas from the disk (Jupiter, Saturn, Uranus, Neptune)
Timescales from young star disks
Protoplanetary disk observations reveal the presence of dust and larger grains through (dust absorbs visible light from the star and re-emits it in the infrared) and millimeter-wavelength emissions
Disk lifetime can be inferred from observations showing a decrease in infrared excess and millimeter emissions over time, with most disks dissipating within a few million years after the star's formation
Planet formation timescales are constrained by the dissipation of the disk, setting an upper limit on the time available for planets to form before the gas and dust are depleted
Observations suggest that planet formation occurs within a few million years of the star's birth, requiring efficient growth and processes
Evidence in circumstellar dust images
Disk structures such as gaps (circular regions with lower dust density) and spiral arms (extended, curved structures) can provide evidence of planet formation
Gaps may indicate the presence of a forming planet that has cleared a path in the disk, while spiral arms can be caused by gravitational instabilities or the presence of a massive planet
Interpreting disk images involves comparing observed structures with theoretical models of planet-disk interactions to constrain the mass and orbit of potential planets
Examples of circumstellar disks with evidence of planet formation include:
: A young star with a disk showing multiple concentric gaps, suggesting the presence of several forming planets
: A disk with a prominent gap at around 1 AU from the star, possibly caused by a forming planet
, composed of dust and small particles, can indicate ongoing planet formation or the presence of fully formed planets
Detection Methods for Exoplanets
: Capturing actual images of planets orbiting other stars, particularly effective for young, bright planets far from their host star
: Measuring the slight wobble of a star caused by the gravitational pull of orbiting planets
: Detecting the periodic dimming of a star's light as a planet passes in front of it
These methods have led to the discovery of numerous , providing evidence for planet formation around other stars
Key Terms to Review (17)
Accretion: Accretion is the process by which particles in space stick together to form larger bodies, such as planets and stars. This occurs through collisions and gravitational attraction, leading to the growth of celestial objects.
Accretion: Accretion is the process by which matter, such as dust, gas, or smaller objects, accumulates over time to form larger bodies, like planets, stars, or galaxies. It is a fundamental mechanism underlying the formation and growth of many celestial objects in the universe.
Circumstellar Dust: Circumstellar dust refers to the fine particulate matter that surrounds a star, often in a disk-like structure. This dust plays a crucial role in the formation and evolution of planetary systems around other stars, providing important evidence for the existence of exoplanets.
Debris Disks: Debris disks are circumstellar disks of dust and debris surrounding a star. They are remnants of the planet formation process and are composed of dust, asteroids, comets, and other rocky and icy bodies that did not coalesce into planets. Debris disks provide important evidence that planets can form around other stars, as their presence suggests the existence of unseen planetary bodies.
Direct Imaging: Direct imaging is a technique used in astronomy to visually observe and capture images of exoplanets, or planets orbiting stars other than our Sun. This method allows astronomers to directly detect the presence of these distant worlds and study their properties, providing valuable insights into planetary systems beyond our own.
Exoplanets: Exoplanets are planets that orbit stars outside our solar system. They vary widely in size, composition, and distance from their parent stars.
Exoplanets: Exoplanets are planets that orbit stars other than our own Sun. These distant worlds provide a window into the diversity of planetary systems beyond our solar system and offer clues about the formation and evolution of planets, including the potential for habitable environments outside Earth.
HL Tau: HL Tau is a young star located in the constellation Taurus, approximately 450 light-years from Earth. It is an important object of study due to its protoplanetary disk, which provides insights into the early stages of planet formation.
HL Tauri: HL Tauri is a young, pre-main sequence star that is part of the Taurus Molecular Cloud, a star-forming region located approximately 450 light-years from Earth. It is considered an important example of evidence that planets form around other stars, as observations of this system have provided insights into the early stages of planetary formation.
Infrared Excess: Infrared excess refers to the observation of additional infrared radiation emitted by a star, beyond what would be expected from the star's photosphere alone. This excess infrared emission is often used as evidence that a planetary system may be forming around the star.
Planetesimals: Planetesimals are small celestial objects that formed from dust and gas in the early solar system. They serve as the building blocks of planets through a process called accretion.
Planetesimals: Planetesimals are the small, rocky or icy bodies that formed the building blocks of the planets in the early stages of the Solar System's development. These objects, ranging from just a few kilometers to hundreds of kilometers in size, gradually accumulated through the process of accretion to eventually create the larger planetary bodies we see today.
Protoplanetary Disk: A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star. It is the birthplace of planets, where the material in the disk begins to coalesce under gravity to form a planetary system.
Radial velocity: Radial velocity is the component of a star's or other celestial object's velocity that is directed along the line of sight of an observer. It can be measured by observing Doppler shifts in the object's spectral lines.
Radial Velocity: Radial velocity refers to the component of an object's velocity that is directed along the line of sight between the object and the observer. It is a crucial concept in astronomy, as it allows for the detection and study of exoplanets, the measurement of stellar properties, and the exploration of the dynamics of our universe.
Transit Method: The transit method is a technique used to detect and study exoplanets, or planets orbiting stars other than our Sun. It involves observing the periodic dimming of a star's brightness as an orbiting planet passes in front of, or transits, the star from the observer's perspective.
TW Hydrae: TW Hydrae is a young pre-main sequence star located in the constellation Hydra, which has been observed to have a protoplanetary disk around it. This disk provides evidence that planets can form around other stars, similar to how our own solar system formed.