Editing Nature (section)

==Beyond Earth==
{{Main|Outer space|Universe|Extraterrestrial life}}

[[File:Planets2013.svg|thumb|left|upright=1.3|[[Planet]]s of the [[Solar System]] ''(sizes to scale, distances and illumination not to scale)'']]
[[File:NGC 4414 (NASA-med).jpg|thumb|'''[[NGC 4414]]''' is a spiral galaxy in the constellation [[Coma Berenices]] about 56,000 [[light-year]]s in diameter and approximately 60 million light-years from [[Earth]].]]

Outer space, also simply called ''space'', refers to the relatively empty regions of the [[Universe]] outside the [[atmosphere]]s of celestial bodies. ''Outer'' space is used to distinguish it from [[airspace]] (and terrestrial locations). There is no discrete boundary between [[atmosphere of Earth|Earth's atmosphere]] and space, as the atmosphere gradually attenuates with increasing altitude. Outer space within the [[Solar System]] is called [[interplanetary medium|interplanetary space]], which passes over into [[interstellar medium|interstellar space]] at what is known as the [[Heliopause (astronomy)|heliopause]].

Outer space is sparsely filled with several dozen types of [[organic chemistry|organic]] [[molecule]]s discovered to date by [[rotational spectroscopy|microwave spectroscopy]], [[cosmic microwave background|blackbody radiation]] left over from the [[Big Bang]] and the origin of the universe, and [[cosmic ray]]s, which include [[ion]]ized [[atomic nucleus|atomic nuclei]] and various [[subatomic particle]]s. There is also some gas, [[plasma (physics)|plasma]] and [[dust]], and small [[meteor]]s. Additionally, there are signs of human life in outer space today, such as material left over from previous crewed and uncrewed launches which are a potential hazard to spacecraft. Some of this [[space debris|debris]] re-enters the atmosphere periodically.

Although Earth is the only body within the [[Solar System]] known to support life, evidence suggests that in the distant past the planet [[Mars]] possessed bodies of liquid water on the surface.<ref>{{cite journal|author = Bibring, J |display-authors=etal|title = Global mineralogical and aqueous mars history derived from OMEGA/Mars Express data|journal = Science|volume = 312|issue = 5772|pages = 400–404|date = 2006|pmid = 16627738|doi = 10.1126/science.1122659|bibcode = 2006Sci...312..400B|s2cid=13968348 |doi-access = }}</ref> For a brief period in Mars' history, it may have also been capable of forming life. At present though, most of the water remaining on Mars is frozen.
If life exists at all on Mars, it is most likely to be located underground where liquid water can still exist.<ref>{{cite web|first = Tariq|last = Malik|date = March 8, 2005|url = http://www.nbcnews.com/id/7129347|title = Hunt for Mars life should go underground|publisher = Space.com via NBC News|access-date = September 4, 2006|df = mdy-all|archive-date = September 30, 2013|archive-url = https://web.archive.org/web/20130930205156/http://www.nbcnews.com/id/7129347/|url-status = dead}}</ref>

Conditions on the other terrestrial planets, [[Mercury (planet)|Mercury]] and [[Venus]], appear to be too harsh to support life as we know it. But it has been conjectured that [[Europa (moon)|Europa]], the fourth-largest moon of [[Jupiter]], may possess a sub-surface ocean of liquid water and could potentially host life.<ref>{{cite web|author = Turner, Scott|date = March 2, 1998|url = http://www2.jpl.nasa.gov/galileo/news8.html|title = Detailed Images From Europa Point To Slush Below Surface|publisher = NASA|access-date = September 28, 2006|archive-url = https://web.archive.org/web/20060929232149/http://www2.jpl.nasa.gov/galileo/news8.html|archive-date = September 29, 2006|url-status = dead|df = mdy-all}}</ref>

Astronomers have started to discover extrasolar [[Earth analog]]s – planets that lie in the [[circumstellar habitable zone|habitable zone]] of space surrounding a [[star]], and therefore could possibly host life as we know it.<ref>Choi, Charles Q. (March 21, 2011) [http://www.space.com/11188-alien-earths-planets-sun-stars.html New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone | Alien Planets, Extraterrestrial Life & Extrasolar Planets | Exoplanets & Kepler Space Telescope] {{Webarchive|url=https://web.archive.org/web/20130703101746/http://www.space.com/11188-alien-earths-planets-sun-stars.html |date=July 3, 2013 }}. Space.com.</ref>