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Stars form in large clouds of gas and dust. Planets grow in the circumstellar disk that surrounds every newborn star.

To learn how stars form, CfA scientists study the structure of dark clouds and the young stars within the clouds. To study the birth of planets, CfA scientists examine the structure of circumstellar disks and outflowing jets associated with the youngest stars. Once a star is mature, its brightness is fairly steady for hundreds of millions to tens billions of years. This steadiness allows planets orbiting a star to develop stable surfaces that might allow life to develop and perhaps flourish. CfA scientists study the structure and evolution of stable and unstable stars and search for planets orbiting around them.

Space Is Full Of Planets, And Most Of Them Don't Even Have Stars

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Stars, Planets and Origins When we look up at the sky at night, we see light produced by stars. Accretion Disks. Binary Stars. Brown Dwarfs. Circumstellar Disks and Planet Formation. Computational Stellar Astrophysics. Dense Cloud Cores. Dynamical Architecture of Planetary Systems.

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Early-type Stars. Embedded Clusters. Evolved Stars. Extrasolar Planets. Giant Magellan Telescope. Hypervelocity Stars. Kozai-Lidov Mechanism. Laser Frequency Combs for Astrophysics astro-combs. Late-Type Stars and Brown Dwarfs. Life and the Cosmos. Magnetohydrodynamic Effects in Hot Planetary Atmospheres;. Massive Star Formation.

Planets & Origins of Life - Webb/NASA

Merging White Dwarfs. Milky Way Halo. Molecular Clouds. Open Clusters. Planet Formation. Planetary Dynamics and Celestial Mechanics. Planetary Nebulae. Resonance Overlap and Planetary Stability.

Star Formation. Alternatively, planets may form in an accretion disk of fallback matter surrounding a pulsar. As stars evolve and turn into red giants , asymptotic giant branch stars, and planetary nebulae they engulf the inner planets, evaporating or partially evaporating them depending on how massive they are. As the star loses mass, planets that are not engulfed move further out from the star. If an evolved star is in a binary or multiple system then the mass it loses can transfer to another star, creating new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets which may also be affected by the mass transfer.

The Solar System consists of an inner region of small rocky planets and outer region of large gas giants. However, other planetary systems can have quite different architectures. Studies suggest that architectures of planetary systems are dependent on the conditions of their initial formation. Theories, such as planetary migration or scattering, have been proposed for the formation of large planets close to their parent stars.

More commonly, systems consisting of multiple Super-Earths have been detected. Some studies suggest that there is at least one planet on average per star. However, the proportion of stars is uncertain because not all planets can yet be detected. The radial-velocity method and the transit method which between them are responsible for the vast majority of detections are most sensitive to large planets in small orbits. Thus, many known exoplanets are "hot Jupiters": planets of Jovian mass or larger in very small orbits with periods of only a few days.

A survey of radial-velocity-detected planets found that about 1. It is known that small planets of roughly Earth-like mass or somewhat larger are more common than giant planets. The authors of this study conclude that "stars are orbited by planets as a rule, rather than the exception". Whatever the proportion of stars with planets, the total number of exoplanets must be very large. Because the Milky Way has at least billion stars, it must also contain tens or hundreds of billions of planets.

Stars, Planets and Origins

Most known exoplanets orbit stars roughly similar to the Sun , that is, main-sequence stars of spectral categories F, G, or K. One reason is that planet-search programs have tended to concentrate on such stars. In addition, statistical analyses indicate that lower-mass stars red dwarfs , of spectral category M are less likely to have planets massive enough to be detected by the radial-velocity method. Stars of spectral categories A and B typically rotate very quickly, which makes it very difficult to measure the small Doppler shifts induced by orbiting planets because the spectral lines are very broad.

However, this type of massive star eventually evolves into a cooler red giant that rotates more slowly and thus can be measured using the radial-velocity method. A few tens of planets have been found around red giants.


Observations using the Spitzer Space Telescope indicate that extremely massive stars of spectral category O, which are much hotter than the Sun, produce a photo-evaporation effect that inhibits planetary formation. Doppler surveys around a wide variety of stars indicate about 1 in 6 stars having twice the mass of the Sun are orbited by one or more Jupiter-sized planets, vs. On the other hand, microlensing surveys indicate that long-period Neptune -mass planets are found around 1 in 3 red dwarfs.

At the low-mass end of star-formation are sub-stellar objects that don't fuse hydrogen: the brown dwarfs and sub-brown dwarfs , of spectral classification L, T and Y. Planets and protoplanetary disks have been discovered around brown dwarfs, and disks have been found around sub-brown dwarfs e. OTS Ordinary stars are composed mainly of the light elements hydrogen and helium. In this study small planets occurred about three times as frequently as large planets around stars of metallicity greater than that of the Sun, but they occurred around six times as frequently for stars of metallicity less than that of the Sun.

The lack of gas giants around low-metallicity stars could be because the metallicity of protoplanetary disks affects how quickly planetary cores can form and whether they accrete a gaseous envelope before the gas dissipates. However, Kepler can only observe planets very close to their star and the detected gas giants probably migrated from further out, so a decreased efficiency of migration in low-metallicity disks could also partly explain these findings. A study found that not only giant planets, but planets of all sizes, have an increased occurrence rate around metal-rich stars compared to metal-poor stars, although the larger the planet, the greater this increase as the metallicity increases.

The study divided planets into three groups based on radius: gas giants, gas dwarfs, and terrestrial planets with the dividing lines at 1. For these three groups, the planet occurrence rates are 9. There is a bias against detecting smaller planets because metal-rich stars tend to be larger, making it more difficult to detect smaller planets, which means that these increases in occurrence rates are lower limits. It has also been shown that stars with planets are more likely to be deficient in lithium.

Most stars form in open clusters , but very few planets have been found in open clusters and this led to the hypothesis that the open-cluster environment hinders planet formation.

Planetary Systems

However, a study concluded that there have been an insufficient number of surveys of clusters to make such a hypothesis. Recent discoveries of both giant planets [31] and low-mass planets [32] in open clusters are consistent with there being similar planet occurrence rates in open clusters as around field stars. After planets, circumstellar disks are one of the most commonly observed properties of planetary systems, particularly of young stars. The Solar System possesses at least four major circumstellar disks the asteroid belt , Kuiper belt , scattered disc , and Oort cloud and clearly observable disks have been detected around nearby solar analogs including Epsilon Eridani and Tau Ceti.