Editing Planetary system (section)

==Galactic distribution of planets==
{{See also|Galactic habitable zone|Extragalactic planet|Globular cluster#Planets}}
[[File:Planet Discovery Neighbourhood in Milky Way Galaxy.jpeg|thumb|300px|90% of planets with known distances are within about 2000 [[light years]] of Earth, as of July 2014.]]
The [[Milky Way]] is 100,000 light-years across, but 90% of planets with known distances are within about 2000 [[light years]] of Earth, as of July 2014. One method that can detect planets much further away is [[microlensing]]. The upcoming [[Nancy Grace Roman Space Telescope]] could use microlensing to measure the relative frequency of planets in the [[galactic bulge]] versus the [[galactic disk]].<ref>[http://exep.jpl.nasa.gov/exopag/exopag9/agenda/jyee_ExoPAG9.pdf SAG 11: Preparing for the WFIRST Microlensing Survey] {{Webarchive|url=https://web.archive.org/web/20140222134148/http://exep.jpl.nasa.gov/exopag/exopag9/agenda/jyee_ExoPAG9.pdf |date=February 22, 2014 }}, Jennifer Yee</ref> So far, the indications are that planets are more common in the disk than the bulge.<ref>[http://science.gsfc.nasa.gov/660/seminars/ASDcolloq/fall2010.html Toward a New Era in Planetary Microlensing] {{Webarchive|url=https://web.archive.org/web/20141103160816/http://science.gsfc.nasa.gov/660/seminars/ASDcolloq/fall2010.html |date=November 3, 2014 }}, Andy Gould, September 21, 2010</ref> Estimates of the distance of microlensing events is difficult: the first planet considered with high probability of being in the bulge is [[MOA-2011-BLG-293Lb]] at a distance of 7.7 kiloparsecs (about 25,000 light years).<ref>{{cite journal | arxiv=1310.3706 | doi=10.1088/0004-637X/780/1/54 | title=Moa-2011-BLG-293Lb: First Microlensing Planet Possibly in the Habitable Zone | date=2013 | last1=Batista | first1=V. | last2=Beaulieu | first2=J. -P. | last3=Gould | first3=A. | last4=Bennett | first4=D. P. | last5=Yee | first5=J. C. | last6=Fukui | first6=A. | last7=Gaudi | first7=B. S. | last8=Sumi | first8=T. | last9=Udalski | first9=A. | journal=The Astrophysical Journal | volume=780 | page=54 }}</ref>

<!--The next two paragraphs contain some material from https://en.wikipedia.org/wiki/Metallicity#Population_I_stars-->  
''Population I'', or ''metal-rich stars'', are those young stars whose [[metallicity]] is highest. The high metallicity of population I stars makes them more likely to possess planetary systems than older populations, because planets form by the [[accretion (astrophysics)|accretion]] of metals.{{citation needed|date=December 2014}} The Sun is an example of a metal-rich star. These are common in the [[spiral arm]]s of the [[Milky Way]].{{citation needed|date=December 2014}} Generally, the youngest stars, the extreme population I, are found farther in and intermediate population I stars are farther out, etc. The Sun is considered an intermediate population I star. Population I stars have regular [[elliptical orbit]]s around the [[Galactic Center]], with a low [[relative velocity]].<ref>{{cite journal| title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect|author=Charles H. Lineweaver |date=2000| doi=10.1006/icar.2001.6607| journal=Icarus| volume=151| issue=2| pages=307–313|arxiv=astro-ph/0012399|bibcode = 2001Icar..151..307L |s2cid=14077895 }}</ref>

''Population II'', or ''metal-poor stars'', are those with relatively low metallicity which can have hundreds (e.g. [[BD +17° 3248]]) or thousands (e.g. [[Sneden's Star]]) times less metallicity than the Sun. These objects formed during an earlier time of the universe.{{citation needed|date=December 2014}} Intermediate population II stars are common in the [[bulge (astronomy)|bulge]] near the center of the [[Milky Way]],{{citation needed|date=December 2014}} whereas Population II stars found in the [[Galactic spheroid#Galactic spheroid|galactic halo]] are older and thus more metal-poor.{{citation needed|date=December 2014}} [[Globular clusters]] also contain high numbers of population II stars.<ref>{{cite journal | author=T. S. van Albada | author2=Norman Baker | title=On the Two Oosterhoff Groups of Globular Clusters | journal=Astrophysical Journal | volume=185 | date=1973 | pages=477–498 | doi=10.1086/152434 | bibcode=1973ApJ...185..477V| doi-access=free }}</ref>
In 2014, the first planets around a halo star were announced around [[Kapteyn's star]], the nearest halo star to Earth, around 13 light years away. However, later research suggests that [[Kapteyn b]] is just an artefact of stellar activity and that Kapteyn c needs more study to be confirmed.<ref>{{cite journal | arxiv=1505.02778 | doi=10.1088/2041-8205/805/2/L22 | title=Stellar Activity Mimics a Habitable-Zone Planet Around Kapteyn's Star | date=2015 | last1=Robertson | first1=Paul | last2=Roy | first2=Arpita | last3=Mahadevan | first3=Suvrath | journal=The Astrophysical Journal | volume=805 | issue=2 | pages=L22 | bibcode=2015ApJ...805L..22R }}</ref> The metallicity of Kapteyn's star is estimated to be about 8<ref group=lower-alpha name="kepteynmetal">[[Metallicity]] of [[Kapteyn's star]] estimated at [Fe/H]= −0.89. 10<sup>−0.89</sup> ≈ 1/8</ref> times less than the Sun.<ref name="kapteyn">{{cite journal | arxiv=1406.0818 | doi=10.1093/mnrasl/slu076 | doi-access=free | title=Two planets around Kapteyn's star: A cold and a temperate super-Earth orbiting the nearest halo red dwarf | date=2014 | last1=Anglada-Escudé | first1=G. | last2=Arriagada | first2=P. | last3=Tuomi | first3=M. | last4=Zechmeister | first4=M. | last5=Jenkins | first5=J. S. | last6=Ofir | first6=A. | last7=Dreizler | first7=S. | last8=Gerlach | first8=E. | last9=Marvin | first9=C. J. | last10=Reiners | first10=A. | last11=Jeffers | first11=S. V. | last12=Butler | first12=R. P. | last13=Vogt | first13=S. S. | last14=Amado | first14=P. J. | last15=Rodríguez-López | first15=C. | last16=Berdiñas | first16=Z. M. | last17=Morin | first17=J. | last18=Crane | first18=J. D. | last19=Shectman | first19=S. A. | last20=Thompson | first20=I. B. | last21=Díaz | first21=M. | last22=Rivera | first22=E. | last23=Sarmiento | first23=L. F. | last24=Jones | first24=H. R. A. | journal=Monthly Notices of the Royal Astronomical Society: Letters | volume=443 | pages=L89–L93 }}</ref>

Different [[Galaxy morphological classification|types of galaxies]] have different histories of [[star formation]] and hence [[Nebular hypothesis#Formation of planets|planet formation]]. Planet formation is affected by the ages, metallicities, and orbits of stellar populations within a galaxy. Distribution of stellar populations within a galaxy varies between the different types of galaxies.<!--"If all galaxies were just like the Milky Way, then the GHZ could just be applied to other galaxies. But, they aren't; there is great variation in their properties. Galaxies differ in their Hubble types (elliptical, spiral, or irregular), metallicities, luminosities, masses, and environments."--><ref>{{cite journal | arxiv=astro-ph/0503298 | doi=10.1007/s11084-005-5010-8 | title=Habitable Zones in the Universe | date=2005 | last1=Gonzalez | first1=Guillermo | journal=Origins of Life and Evolution of Biospheres | volume=35 | issue=6 | pages=555–606 | pmid=16254692 | bibcode=2005OLEB...35..555G }}</ref>
Stars in [[elliptical galaxy|elliptical galaxies]] are much older than stars in [[spiral galaxy|spiral galaxies]]. Most elliptical galaxies contain mainly [[stellar evolution#Low-mass stars|low-mass stars]], with minimal [[star formation|star-formation]] activity.<ref name="author">John, D, (2006), ''Astronomy'', {{ISBN|1-4054-6314-7}}, p. 224-225</ref> The distribution of the different types of galaxies in the [[universe]] depends on their location within [[galaxy cluster]]s, with elliptical galaxies found mostly close to their centers.<ref>{{cite journal |author=Dressler, A. |date=March 1980 |title=Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies. |journal=The Astrophysical Journal |volume=236 |pages=351–365 |bibcode=1980ApJ...236..351D |doi=10.1086/157753|doi-access=free }}</ref>