Planet-hosting star

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Planet-hosting stars are stars which host planets, therefore forming planetary systems.

Contents

This article describes the correlations between stars' characteristics and the characteristics of the planets that orbit them, and other connections between stars and their planets.

Proportion of stars with planets

Most stars are accompanied by planets, though the exact proportion remains uncertain due to current limitations in detecting distant exoplanets. Current research calculates that there is, on average, at least one planet per star. [1] [2] One in five Sun-like stars [lower-alpha 1] is expected to have an "Earth-sized" [lower-alpha 2] planet in the habitable zone. The radial-velocity method and the transit method (the two methods responsible for the vast majority of detected planets) 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 from 2005 on radial-velocity-detected planets found that about 1.2% of Sun-like stars have a 'Hot Jupiter', where "Sun-like star" refers to any main-sequence star of spectral classes late-F, G, or early-K without a close stellar companion. [3] This 1.2% is more than double the frequency of 'Hot Jupiters' detected by the Kepler spacecraft, for which a possible reason is that the Kepler field of view is covering a different region of the Milky Way where the metallicity of stars is different. [4] It is further estimated that 3% to 4.5% of Sun-like stars possess a giant planet with an orbital period of 100 days or less, where "giant planet" means a planet of at least 30 Earth masses. [5]

It is known that small planets (of roughly Earth-like mass or slightly larger) are more common than giant planets. [6] It also appears that there are more planets in large orbits than in small orbits. Based on this, it is estimated that about 20% of Sun-like stars have at least one giant planet, whereas at least 40% may have planets of lower mass. [5] [7] [8] A 2012 study of gravitational microlensing data collected between 2002 and 2007 concludes the proportion of stars with planets is much higher and estimates an average of 1.6 planets orbiting between 0.5 and 10 AU per star in the Milky Way. The authors of this study conclude that "stars are orbited by planets as a rule, rather than the exception". [2] In November 2013, it was announced that 22±8% of Sun-like [lower-alpha 1] stars have an Earth-sized [lower-alpha 2] planet in the habitable [lower-alpha 3] zone. [9] [10]

Regardless of the proportion of stars with planets, the total number of exoplanets must be very large. Since the Milky Way has at least 100 billion stars, it should also contain tens or hundreds of billions of planets.

Type of star, spectral classification

The Morgan-Keenan spectral classification Morgan-Keenan spectral classification.svg
The Morgan-Keenan spectral classification

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. [5] [11] Nevertheless, many planets around red dwarfs have been discovered by the Kepler spacecraft by the transit method, which can detect smaller planets.

Stars of spectral category A 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. [12] 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. [12] 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. [13] When the O-type star goes supernova any planets that had formed would become free-floating due to the loss of stellar mass unless the natal kick of the resulting remnant pushes it in the same direction as an escaping planet. [14] Fallback disks of matter that failed to escape orbit during a supernova may form planets around neutron stars and black holes. [15]

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. 1 in 16 for Sun-like stars and only 1 in 50 for red dwarfs. On the other hand, microlensing surveys indicate that long-period Neptune-mass planets are found around 1 in 3 red dwarfs. [16] Kepler Space Telescope observations of planets with up to one year periods show that occurrence rates of Earth- to Neptune-sized planets (1 to 4 Earth radii) around M, K, G, and F stars are successively higher towards cooler, less massive stars. [17]

At the low-mass end of star-formation are sub-stellar objects that do not 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.g. OTS 44).

Rogue planets ejected from their system could retain a system of satellites. [18]

Metallicity

Ordinary stars are composed mainly of the light elements hydrogen and helium. They also contain a small proportion of heavier elements, and this fraction is referred to as a star's metallicity (even if the elements are not metals in the traditional sense), [3] denoted [m/H] and expressed on a logarithmic scale where zero is the Sun's metallicity. Stars with a higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.

A 2012 study of the Kepler spacecraft data found that smaller planets, with radii smaller than Neptune's were found around stars with metallicities in the range −0.6 < [m/H] < +0.5 (about four times less than that of the Sun to three times more), [lower-alpha 4] whereas larger planets were found mostly around stars with metallicities at the higher end of this range (at solar metallicity and above). 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. [19]

A 2014 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 1.7 and 3.9 Earth radii. For these three groups the planet occurrence rates are 9.30, 2.03, and 1.72 times higher for metal-rich stars than for metal-poor stars, respectively. 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. [20]

It has also been shown that Sun-like stars with planets are much more likely to be deficient in lithium, although this correlation is not seen at all in other types of stars. [21] However, this claimed relationship has become a point of contention in the planetary astrophysics community, being frequently denied [22] [23] but also supported. [24] [25]

Multiple stars

Stellar multiplicity increases with stellar mass: the likelihood of stars being in multiple systems is about 25% for red dwarfs, about 45% for Sun-like stars, and rises to about 80% for the most massive stars. Of the multiple stars about 75% are binaries and the rest are higher-order multiplicities. [26]

More than one hundred planets have been discovered orbiting one member of a binary star system (e.g. 55 Cancri, possibly Alpha Centauri Bb), [27] and several circumbinary planets have been discovered which orbit around both members of a binary star (e.g. PSR B1620-26 b, Kepler-16b). A few dozen planets in triple star systems are known (e.g. 16 Cygni Bb) [28] and two in quadruple systems Kepler 64 and 30 Arietis. [29]

The Kepler results indicate circumbinary planetary systems are relatively common (as of October 2013 the spacecraft had found seven circumbinary planets out of roughly 1000 eclipsing binaries searched). One puzzling finding is that although half of the binaries have an orbital period of 2.7 days or less, none of the binaries with circumbinary planets have a period less than 7.4 days. Another surprising Kepler finding is circumbinary planets tend to orbit their stars close to the critical instability radius (theoretical calculations indicate the minimum stable separation is roughly two to three times the size of the stars' separation). [30]

In 2014, from statistical studies of searches for companion stars, it was inferred that around half of exoplanet host stars have a companion star, usually within 100AU. [31] [32] This means that many exoplanet host stars that were thought to be single are binaries, so in many cases it is not known which of the stars a planet actually orbits, and the published parameters of transiting planets could be significantly incorrect because the planet radius and distance from star are derived from the stellar parameters. Follow-up studies with imaging (such as speckle imaging) are needed to find or rule out companions (and radial velocity techniques would be required to detect binaries really close together) and this has not yet been done for most exoplanet host stars. Examples of known binary stars where it is not known which of the stars a planet orbits are Kepler-132 and Kepler-296, [33] although a 2015 study found that the Kepler-296 planets were likely orbiting the brighter star. [34]

Open clusters

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. However, a 2011 study concluded that there have been an insufficient number of surveys of clusters to make such a hypothesis. [35] The lack of surveys was because there are relatively few suitable open clusters in the Milky Way. Recent discoveries of both giant planets [36] and low-mass planets [37] in open clusters are consistent with there being similar planet occurrence rates in open clusters as around field stars.

The open cluster NGC 6811 contains two known planetary systems Kepler-66 and Kepler-67.

Age

Asteroseismology

Stellar activity

Further reading

Related Research Articles

<span class="mw-page-title-main">Exoplanet</span> Planet outside the Solar System

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 1 March 2024, there are 5,640 confirmed exoplanets in 4,155 planetary systems, with 895 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">CoRoT</span> European space telescope that operated between 2006 - 2014

CoRoT was a space telescope mission which operated from 2006 to 2013. The mission's two objectives were to search for extrasolar planets with short orbital periods, particularly those of large terrestrial size, and to perform asteroseismology by measuring solar-like oscillations in stars. The mission was led by the French Space Agency (CNES) in conjunction with the European Space Agency (ESA) and other international partners.

<span class="mw-page-title-main">Planetary system</span> Set of non-stellar objects in orbit around a star

A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consist of bodies such as dwarf planets, asteroids, natural satellites, meteoroids, comets, planetesimals and circumstellar disks. The Sun together with the planetary system revolving around it, including Earth, forms the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems.

<span class="mw-page-title-main">Exomoon</span> Moon beyond the Solar System

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.

<span class="mw-page-title-main">Methods of detecting exoplanets</span>

Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets reported as of January 2024 have been observed directly, with even fewer being resolved from their host star.

<span class="mw-page-title-main">Subdwarf B star</span> Subdwarf star with spectral type B - extremely hot small star

A B-type subdwarf (sdB) is a kind of subdwarf star with spectral type B. They differ from the typical subdwarf by being much hotter and brighter. They are situated at the "extreme horizontal branch" of the Hertzsprung–Russell diagram. Masses of these stars are around 0.5 solar masses, and they contain only about 1% hydrogen, with the rest being helium. Their radius is from 0.15 to 0.25 solar radii, and their surface temperature is from 20,000 to 40,000 K.

<span class="mw-page-title-main">Circumbinary planet</span> Planet that orbits two stars instead of one

A circumbinary planet is a planet that orbits two stars instead of one. The two stars orbit each other in a binary system, while the planet typically orbits farther from the center of the system than either of the two stars. In contrast, circumstellar planets in a binary system have stable orbits around one of the two stars, closer in than the orbital distance of the other star. Studies in 2013 showed that there is a strong hint that a circumbinary planet and its stars originate from a single disk.

<span class="mw-page-title-main">Discoveries of exoplanets</span> Detecting planets located outside the Solar System

An exoplanet is a planet located outside the Solar System. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such until 2016; no planet discovery has yet come from that evidence. What turned out to be the first detection of an exoplanet was published among a list of possible candidates in 1988, though not confirmed until 2003. The first confirmed detection came in 1992, with the discovery of terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. As of 1 March 2024, there are 5,640 confirmed exoplanets in 4,155 planetary systems, with 895 systems having more than one planet. This is a list of the most notable discoveries.

<span class="mw-page-title-main">Kepler-16b</span> Gas giant orbiting Kepler-16 star system

Kepler-16b is an exoplanet. It is a Saturn-mass planet consisting of half gas and half rock and ice, and it orbits a binary star, Kepler-16, with a period of 229 days. "[It] is the first confirmed, unambiguous example of a circumbinary planet – a planet orbiting not one, but two stars," said Josh Carter of the Center for Astrophysics | Harvard & Smithsonian, one of the discovery team.

Kepler-70, also known as KIC 5807616 and KOI-55, is a star about 3,600 light-years away in the constellation Cygnus, with an apparent visual magnitude of 14.87. This is too faint to be seen with the naked eye; viewing it requires a telescope with an aperture of 40 cm (20 in) or more. A subdwarf B star, Kepler-70 passed through the red giant stage some 18.4 million years ago. In its present-day state, it is fusing helium in its core. Once it runs out of helium it will contract to form a white dwarf. It has a relatively small radius of about 0.2 times the Sun's radius; white dwarfs are generally much smaller. The star may be host to a planetary system with two planets, although later research indicates that this is not in fact the case.

Kepler-47 is a binary star system in the constellation Cygnus located about 3,420 light-years away from Earth. The stars have three exoplanets, all of which orbit both stars at the same time, making this a circumbinary system. The first two planets announced are designated Kepler-47b, and Kepler-47c, and the third, later discovery is Kepler-47d. Kepler-47 is the first circumbinary multi-planet system discovered by the Kepler mission. The outermost of the planets is a gas giant orbiting within the habitable zone of the stars. Because most stars are binary, the discovery that multi-planet systems can form in such a system has impacted previous theories of planetary formation.

<span class="mw-page-title-main">Kepler-47c</span> Kepler-47c is a gas giant.

Kepler-47c is an exoplanet orbiting the binary star system Kepler-47, the outermost of three such planets discovered by NASA's Kepler spacecraft. The system, also involving two other exoplanets, is located about 3,400 light-years away.

Kepler-102 is a star 353 light-years away in the constellation of Lyra. Kepler-102 is less luminous than the Sun. The star system does not contain any observable amount of dust. Kepler-102 is suspected to be orbited by a binary consisting of two red dwarf stars, at projected separations of 591 and 627 AU.

Kepler-432 is a binary star system with at least two planets in orbit around the primary companion, located about 2,830 light-years away from Earth.

References

  1. 1 2 For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars was not available so this statistic is an extrapolation from data about K-type stars
  2. 1 2 For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
  3. ↑ For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
  4. ↑ Converting log scale [m/H] to multiple of solar metallicity: [(10−0.6 ≈ 1/4), (100.5 ≈ 3)]
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  33. ↑ Validation of Kepler's Multiple Planet Candidates. II: Refined Statistical Framework and Descriptions of Systems of Special Interest, Jack J. Lissauer, Geoffrey W. Marcy, Stephen T. Bryson, Jason F. Rowe, Daniel Jontof-Hutter, Eric Agol, William J. Borucki, Joshua A. Carter, Eric B. Ford, Ronald L. Gilliland, Rea Kolbl, Kimberly M. Star, Jason H. Steffen, Guillermo Torres, (Submitted on 25 Feb 2014)
  34. ↑ The Five Planets in the Kepler-296 Binary System All Orbit the Primary: A Statistical and Analytical Analysis, Thomas Barclay, Elisa V. Quintana, Fred C. Adams, David R. Ciardi, Daniel Huber, Daniel Foreman-Mackey, Benjamin T. Montet, Douglas Caldwell, 7 May 2015
  35. ↑ Ensemble analysis of open cluster transit surveys: upper limits on the frequency of short-period planets consistent with the field, Jennifer L. van Saders, B. Scott Gaudi, (Submitted on 15 Sep 2010)
  36. ↑ Three planetary companions around M67 stars, A. Brucalassi (1,2), L. Pasquini (3), R. Saglia (1,2), M. T. Ruiz (4), P. Bonifacio (5), L. R. Bedin (6), K. Biazzo (7), C. Melo (8), C. Lovis (9), S. Randich (10) ((1) MPI Munich, (2) UOM-LMU Munchen, (3) ESO Garching, (4) Astron. Dpt. Univ. de Chile, (5) GEPI Paris, (6) INAF-OAPD, (7) INAF-OACT, (8) ESO Santiago, (9) Obs. de Geneve, (10) INAF-OAFI) (Submitted on 20 Jan 2014)
  37. ↑ The same frequency of planets inside and outside open clusters of stars, Søren Meibom, Guillermo Torres, Francois Fressin, David W. Latham, Jason F. Rowe, David R. Ciardi, Steven T. Bryson, Leslie A. Rogers, Christopher E. Henze, Kenneth Janes, Sydney A. Barnes, Geoffrey W. Marcy, Howard Isaacson, Debra A. Fischer, Steve B. Howell, Elliott P. Horch, Jon M. Jenkins, Simon C. Schuler & Justin Crepp Nature 499, 55–58 (04 July 2013) doi:10.1038/nature12279 Received 06 November 2012 Accepted 02 May 2013 Published online 26 June 2013
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