Query: We have also been told the problem is DEFINITELY NOT a billions-year-old planet running through cycles where the temperature might fluctuate a bit.

Relevant passages:
Earth <=SEP=> Earth ( from Eorðe -LSB- Γαῖα , Gaia -RSB- Terra ) , otherwise known as the World or the Globe , is the third planet from the Sun and the only object in the Universe known to harbor life . It is the densest planet in the Solar System and the largest of the four terrestrial planets .   According to radiometric dating and other sources of evidence , Earth formed about 4.54 billion years ago . Earth 's gravity interacts with other objects in space , especially the Sun and the Moon , Earth 's only natural satellite . During one orbit around the Sun , Earth rotates about its axis over 365 times ; thus , an Earth year is about 365.26 days long . Earth 's axis of rotation is tilted , producing seasonal variations on the planet 's surface . The gravitational interaction between the Earth and Moon causes ocean tides , stabilizes the Earth 's orientation on its axis , and gradually slows its rotation .   Earth 's lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years . About 71 % of Earth 's surface is covered with water , mostly by its oceans . The remaining 29 % is land consisting of continents and islands that together have many lakes , rivers and other sources of water that contribute to the hydrosphere . The majority of Earth 's polar regions are covered in ice , including the Antarctic ice sheet and the sea ice of the Arctic ice pack . Earth 's interior remains active with a solid iron inner core , a liquid outer core that generates the Earth 's magnetic field , and a convecting mantle that drives plate tectonics .   Within the first billion years of Earth 's history , life appeared in the oceans and began to affect the Earth 's atmosphere and surface , leading to the proliferation of aerobic and anaerobic organisms . Some geological evidence indicates that life may have arisen as much as 4.1 billion years ago . Since then , the combination of Earth 's distance from the Sun , physical properties , and geological history have allowed life to evolve and thrive . In the history of the Earth , biodiversity has gone through long periods of expansion , occasionally punctuated by mass extinction events . Over 99 % of all species that ever lived on Earth are extinct . Estimates of the number of species on Earth today vary widely ; most species have not been described . Over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival . Humans have developed diverse societies and cultures ; politically , the world has about 200 sovereign states .

Planetary nebula <=SEP=> A planetary nebula , often abbreviated as PN or plural PNe , is a kind of emission nebula consisting of an expanding , glowing shell of ionized gas ejected from old red giant stars late in their lives . The word `` nebula '' is Latin for mist or cloud , and the term `` planetary nebula '' is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope , these objects resemble the rounded shapes of planets . Herschel 's name for these objects was popularly adopted and has not been changed . They are a relatively short-lived phenomenon , lasting a few tens of thousands of years , compared to a typical stellar lifetime of several billion years .   A mechanism for formation of most planetary nebulae is thought to be the following : at the end of the star 's life , during the red-giant phase , the outer layers of the star are expelled by strong stellar winds . After most of the red giant 's atmosphere is dissipated , the ultraviolet radiation of the hot luminous core , called a planetary nebula nucleus ( PNN ) , ionizes the outer layers earlier ejected from the star . Absorbed ultraviolet light energises the shell of nebulous gas around the central star , causing it to appear as a brightly coloured planetary nebula .   Planetary nebulae likely play a crucial role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created . Planetary nebulae are also observed in more distant galaxies , yielding useful information about their chemical abundances .   Starting from the 1990s , Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies . About one-fifth are roughly spherical , but the majority are not spherically symmetric . The mechanisms that produce such a wide variety of shapes and features are not yet well understood , but binary central stars , stellar winds and magnetic fields may play a role .

Planetary habitability <=SEP=> Planetary habitability is the measure of a planet 's or a natural satellite 's potential to have habitable environments hospitable to life . Habitable environments do not need to contain life . Life may develop directly on a planet or satellite or be transferred to it from another body , a hypothetical process known as panspermia . As the existence of life beyond Earth is unknown , planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life 's flourishing -- in particular those factors that have sustained complex , multicellular organisms and not just simpler , unicellular creatures . Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology .   An absolute requirement for life is an energy source , and the notion of planetary habitability implies that many other geophysical , geochemical , and astrophysical criteria must be met before an astronomical body can support life . In its astrobiology roadmap , NASA has defined the principal habitability criteria as `` extended regions of liquid water , conditions favorable for the assembly of complex organic molecules , and energy sources to sustain metabolism '' .   In determining the habitability potential of a body , studies focus on its bulk composition , orbital properties , atmosphere , and potential chemical interactions . Stellar characteristics of importance include mass and luminosity , stable variability , and high metallicity . Rocky , wet terrestrial-type planets and moons with the potential for Earth-like chemistry are a primary focus of astrobiological research , although more speculative habitability theories occasionally examine alternative biochemistries and other types of astronomical bodies .   The idea that planets beyond Earth might host life is an ancient one , though historically it was framed by philosophy as much as physical science.This article is an analysis of planetary habitability from the perspective of contemporary physical science . A historical viewpoint on the possibility of habitable planets can be found at Beliefs in extraterrestrial life and Cosmic pluralism . For a discussion of the probability of alien life see the Drake equation and Fermi paradox . Habitable planets are also a staple of fiction ; see Planets in science fiction . The late 20th century saw two breakthroughs in the field . The observation and robotic spacecraft exploration of other planets and moons within the Solar System has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies . The discovery of extrasolar planets , beginning in the early 1990s and accelerating thereafter , has provided further information for the study of possible extraterrestrial life . These findings confirm that the Sun is not unique among stars in hosting planets and expands the habitability research horizon beyond the Solar System .   The chemistry of life may have begun shortly after the Big Bang , 13.8 billion years ago , during a habitable epoch when the Universe was only 10 -- 17 million years old . According to the panspermia hypothesis , microscopic life -- distributed by meteoroids , asteroids and other small Solar System bodies -- may exist throughout the Universe . Nonetheless , Earth is the only place in the Universe known to harbor life . Estimates of habitable zones around other stars , along with the discovery of hundreds of extrasolar planets and new insights into the extreme habitats here on Earth , suggest that there may be many more habitable places in the Universe than considered possible until very recently . On 4 November 2013 , astronomers reported , based on Kepler space mission data , that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way . 11 billion of these estimated planets may be orbiting Sun-like stars . The nearest such planet may be 12 light-years away , according to the scientists .