Earth analog

The planet Earth

An Earth analog (also referred to as a Twin Earth, Earth Twin, Second Earth, Alien Earth, Earth 2 or Earth-like planet) is a theoretical other planet (or world) with conditions similar to Earth.

The possibility is of particular interest to humans as it is easily inferred that the more similar a planet is to Earth, the more likely it is of sustaining Earth-like complex extraterrestrial life and more importantly, civilization. As such it has long been speculated and the subject expressed in science, philosophy, science fiction and popular culture.

Advocates of space colonization have long sought an Earth Analog as a "second home" while advocates for space and survival would regard such a planet as a potential "new home".

Some scientific theories speculate that Earth analogs, such as ancient Mars and Venus may have existed in our Solar System in the past and that in the distant future Titan may naturally become one or that technology may be used to artificially produce one by means of terraforming, virtual reality or simulated reality.

Technological advances in extrasolar planet detection are rapidly refining the probabilities of a real discovery, not just in this universe, but in the present Milky Way galaxy. Although the distribution and attributes of the studied planets are still very much unknown, current calculations suggest that they may be relatively common in the universe. 2011 estimates by the NASA's Jet Propulsion Laboratory (JPL) based on observations from the Kepler mission determined that about "1.4 to 2.7 percent" of all sun-like stars are expected to have earthlike planets "within the habitable zones of their stars". This means there are "two billion" of them in our own Milky Way galaxy alone and assuming that all galaxies have a similar number as the Milky Way, in the 50 billion galaxies in the observable universe there may be as many as a sextillion.

The mediocrity principle suggests that there is a chance that serendipitous events may have allowed an Earth-like planet to form elsewhere that would allow the emergence of complex, multi-cellular life. Philosophers have pointed out that the size of the universe is such that a near identical planet must exists somewhere, such theories include the philosophy of Multiverse and the Twin Earth thought experiment. However, the Rare Earth hypothesis asserts that if the strictest criteria are applied, a true Earth Analog, should such a planet exist may be so far away that humans may never locate it.

The more recent serious scientific findings have greatly influenced the scope of the fields of astrobiology, models of planetary habitability and Search for Extraterrestrial Intelligence. Astrobiologists assert that Earth analogs would most likely be found in a stellar habitable zone in which liquid water could exist, providing also the conditions for supporting life. Some astrobiologists, such as Dirk Schulze-Makuch have theorised that a sufficiently massive natural satellite may form a habitable moon similar to Earth. NASA and the SETI Institute have proposed categorising the increasing number of Earth-like planets found using a measure called the Earth Similarity Index (ESI) based on mass, radius and temperature. According to this measure, the confirmed planets most similar to Earth are Gliese 667C c (0.85), Kepler 22b and HD 85512 b (0.79), Gliese 581 d (0.74), Mars and Gliese 581c (0.70).

History

Around 400BC, Philolaus proposed that there existed a Counter-Earth, a counter-balance of the same mass to prevent the universe from being flung apart.

Between 1858 and 1920, Mars was believed by many, including some scientists, to be very similar to Earth, only drier with a thick atmosphere, similar axial tilt, orbit and seasons as well as a Martian civilisation that had built great Martian canals. These theories were advanced by Giovanni Schiaparelli, Percival Lowell and others. As such Mars in fiction portrayed the red planet as similar to Earth but with a desert like landscape. Images and data from the Mariner and Viking space probes, however, portrayed the planet as a dead cratered world. Although with continuing discoveries many Earth comparisons remained. The Mars Ocean Hypothesis, having had its seeds sown from the Viking missions and popularised during the 1980s, speculation began that Mars was once much more Earth-like and could have actually given origin to life.

Likewise, until the 1960s, Venus was believed by many, including some scientists, to be a warmer version of Earth with a thick atmosphere and either hot and dusty or humid with water clouds and oceans. Venus in fiction was often portrayed as having similarities to Earth and many speculated about Venusian civilisation. These beliefs were dispelled in the 1960s as the first space probes gathered more accurate scientific data on the planet and found that Venus is a hellish world with the surface temperature around 900 °F (482 °C) under the atmosphere 92 times thicker the Earth's.

Attributes and Criteria

The probability of finding an Earth analog depends mostly on the attributes which are expected to be similar, and these vary greatly. Generally it is considered that it would be a terrestrial planet and there have been several scientific studies aimed at finding such planets. Often implied but not limited to are such criteria as planet size, star size and type (i.e. Solar analog), orbital distance and stability, axial tilt and rotation, similar geography, oceans, air and weather conditions, strong magnetosphere and even the presence Earth-like complex life (possibly through convergent evolution or parallel evolution). If there is complex life, there could be some forests covering much of the land. If there is intelligent life, some parts of land could be covered in cities. Some things that are assumed of such a planet may be unlikely due to Earth's own history. For instance the Earth's atmosphere was not always oxygen-rich and this is a biosignature from the emergence of photosynthetic life. The formation, presence, influence on these characteristics of the moon (such as tidal forces) may also pose a problem in finding an Earth analog.

Size

Size Comparisons: Kepler-20e and Kepler-20f with Venus and Earth.

Size is often thought to be an effective measure as planets of Earth's size are thought more likely to be terrestrial in nature and be capable of retaining a significant atmosphere.

The closest planets and natural satellites to Earth's size are:

Name	Earth Masses (Me)	Earth Radii (Re)	Note
Kepler-22b	< 36	2.4	Much larger. Within the HZ of solar analog.
Gliese 667 Cc	4.5	-	Within the HZ of red dwarf. Significantly less massive than Kepler-22b.
55 Cancri e	8.63	2	much larger
Kepler-9d	?	1.64	larger
COROT-7b	<9	1.58	larger
Kepler-10b	4.56	1.416	larger
Kepler-20f	< 14.3	1.03	slightly larger and likely more massive
Earth	1	1
Venus	0.815	0.949	slightly smaller
Kepler-20e	< 3.08	0.87	smaller
KOI-961.01		0.78	smaller
KOI-961.02		0.73	smaller
KOI-961.03		0.57	much smaller
Mars	0.107	0.533	much smaller
Ganymede	0.025	0.413	much smaller
Titan	0.0225	0.404	much smaller

This comparison indicates that size alone is a poor measure, particularly in terms of habitability. Temperature must also be considered as Venus and the planets of Kepler-20 (discovered in 2011), COROT-7b and the three planets of KOI-961 (confirmed in 2012) are incredibly hot and Mars, Ganymede and Titan are frigid worlds, resulting also in wide variety of surface and atmospheric conditions. The mass of the Solar System's moons are a tiny fraction of that of Earth whereas the mass of extrasolar planets are very difficult to accurately measure. However discoveries of Earth-sized exoplanets are important as they may indicate the probable frequency and distribution of Earth-like planets. One of the first steps to finding more Earth-like planets may be shortlisting candidates of similar size and then comparing their temperatures to that of Earth.

Terrestrial

Although cold and dry, much of the surface and topography of Mars bears similarities to that of arid regions on Earth

Another criteria often cited is that an Earth analog must be terrestrial, that is, it should possess a similar surface geology - a planetary surface composed of similar surface materials. The closest known example is Mars and Titan and while there are similarities in their types of landforms and surface compositions there are also significant differences.

Many of Earth's surface materials and landforms are formed as a result of interaction with water (such as clay and sedimentary rocks) or as a byproduct of organic life (such as limestone or coal), interaction with the atmosphere, volcanically or artificially.

A true Earth analog therefore might need to have formed through similar processes, having possessed an atmosphere, volcanic interactions with the surface, past or present liquid water and lifeforms.

The Geography of Mars and of Titan in particular appear to include many features similar to Earth.

Temperature

There are several factors that can determine planetary temperatures and therefore several measures that can draw direct comparisons to that of the Earth in planets where atmospheric conditions are unknown.. Equilibrium temperature is used for planets without atmospheres. With atmospheres, a greenhouse effect is assumed. Finally surface temperature is used. Each of these temperatures is of course affected by climate which is influenced by the orbit and rotation (or tidal locking) of the planet, each which introduces further variabilities.

Below is a comparison of the confirmed planets with the closest known temperatures to Earth.

Temperature comparisons	Venus	Earth	Kepler 22b	Mars
Global Equilibrium Temperature	307 K 34 °C 93 °F	255 K −18 °C −0.4 °F	262 K −11 °C 22.2 °F	206 K −67 °C −88.6 °F
+ Venus' GHG effect	737 K 464 °C 867 °F
+ Earth's GHG effect		288 K 15 °C 59 °F	295 K 22 °C 71.6 °F
+ Mars' GHG effect				210 K −63 °C −81 °F
Tidally locked	Almost	No	Unknown	No
Global Bond Albedo	0.9	0.29	Unknown	0.25
Refs.

Solar Analog

Main article: Solar analog

Another criteria of an Earth Analog is that it should orbit a Solar Analog, that is, a star much like our own sun, similar photometrically or in terms of spectral type. It follows that the composition of a planet around a Solar Twin with similar metallicity might have a similar composition to Earth. Also this would help eliminate stellar extremes and variability that may be applicable to other types of stars.

While planets have been discovered orbiting similar stars to the Sun, most are gas giant or Super-Earth sized, and additionally many of these planetary systems have proven to be surprisingly different to our own.

Kepler-22, the parent star of Kepler-22b is slightly smaller and cooler than the Sun.

This measure is not entirely reliable as Mars and Venus also orbit the Sun but have different compositions and properties.

Surface-water and hydrological cycle

Water covers 70% of Earth's surface and is required by all known life.

Kepler-22b, located in the habitable zone of a Sun-like star may be the best exoplanetary candidate for extraterrestrial surface water discovered to date, but is significantly larger than Earth and its actual composition is unknown

Main article: Habitable zone

Main article: Mars Ocean Hypothesis

The concept of the Liquid Water Zone (or habitable zone) defines a region where water can exist on the surface and based on the properties of both the Earth and Sun. Under this model, Earth orbits roughly at the centre of this zone or in the "Goldilocks" position. The Earth is the only planet in the universe confirmed to possess large bodies of surface water. Venus, is on the hot side of the zone while Mars is on the cold side, neither are known to have persistent surface water, though evidence exists that Mars and it is speculated that Venus did have in their ancient pasts. Thus extrasolar planets (or moon) in the Goldilocks position with substantial atmospheres may more possess oceans and water clouds like those on Earth.

Some argue that a true Earth Analog must not only have a similar position of its planetary system but also orbit a Solar Analog and have a near circular orbit such that it remains continually habitable like the Earth.

The best candidates to date by these measure is Kepler-22b, it orbits a Sun-like star in a similar position within the habitable zone, however it is much larger than Earth and its composition is currently unknown. Our Solar System has shown that just being in the habitable zone does not guarantee that a planet is "Earth-like".

In addition to surface water, a true Earth-Analog would require a mix of oceans or lakes and areas not covered by water, or land.

However planets remarkably Earth-like may be found using less strict criteria. For example under certain conditions, such as strong greenhouse effect, atmospheric pressure or gravity, a planet could theoretically possess surface lakes and oceans without being strictly within the habitable zone. Indeed, Saturn's moon Titan is the only other body in the Solar System known to have surface liquid, however it is hydrocarbons rather than water. Other worlds may be similar to Earth, but have a largely dry or icy surface but with atmospheric water vapour, ground water or subsurface ocean, much like some arid and polar regions on Earth.

The Mars Ocean Hypothesis states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet’s geologic history. This primordial ocean, dubbed Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere, a region which lies 4–5 km (2.5–3 miles) below the mean planetary elevation, at a time period of approximately 3.8 billion years ago. Evidence for this ocean includes geographic features resembling ancient shorelines, and the chemical properties of the Martian soil and atmosphere. Early Mars would have required a denser atmosphere and warmer climate to allow liquid water to remain at the surface.

A younger Venus is believed to have possessed Earth-like oceans, but these evaporated as the temperature rose. This may be due to the fact that Venus, owing to its slow rotation, does not have a significant magnetic field, allowing the constituent atoms of the water to be blown away by the solar wind.

Axial Tilt and Rotation

Earth's axial tilt is 23.44°.

The Axial Tilt of Earth is believed to play a large role in stabilising climate and is the cause of seasonal changes in climate. The Earth's axial tilt varies between 22.1° and 24.5° and is stabilized by the Moon. In addition to this steady decrease there are much smaller short term (18.6 years) variations, known as nutation, mainly due to the changing plane of the moon's orbit. This can shift the Earth's axial tilt by plus or minus 0.005 degree.

The closest axial tilt of a terrestrial planet is Mars. Though its tilt is within a similar range, its range is also less stable, believed to be between 11° and 49° as a result of gravitational perturbations from other planets. Mars like Earth experiences seasons. Venus, however has a miniscule axial tilt.

Earth's rotation creates a day/night cycle. The day of Mars is very similar to Earth's while other terrestrial planets, such as Venus and Mercury have extremely long days in comparison due to tidal forces.

Terraforming

Main article: Terraforming

Artist's conception of a terraformed Venus, a potential Earth analog

Terraforming (literally, "Earth-forming") of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth to make it habitable by terran organisms.

Due to proximity and similarity in size, Mars and to a lesser extent Venus are seen as the most likely candidates for terraforming.