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| An imagined habitable exoplanet |
Scientists are hot on the trail
of habitable, earth-like planets. So what are such planets? They are
places where human beings could live without any genetic manipulation. Higher
forms of life characteristic of earth should thrive there.
Habitability requires a planet large enough to maintain a
gravitational hold on its atmosphere, yet not so large as to generate a
crushing atmospheric pressure. Planetary gravity should not be so weak
that our muscles and bones atrophy, yet not so strong that easy movement
is impossible.
Average temperature should result in liquid water as the
normal state, with oceans on the planetary surface. Plate tectonics
should assure a stable carbon concentration in the atmosphere,
preventing a runaway increase in temperature.
Large asteroids should not
be constantly plunging through the atmosphere and striking the
planetary surface. Radiation should not cause life to mutate
incessantly, thus the need for a strong magnetic field to deflect the
solar wind. The planet should not be tidally locked with its primary
star such that one hemisphere always faces its sun and one is in
perpetual darkness.
Reputable sources regarding habitable exoplanets include: Habitable Exoplanets Catalog, Habitable Zone Gallery, NASA and Planetary Biology, the latter with a sortable list of exoplanets, including habitable zone candidates.
Habitable Zone
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| Chart showing location of a star's habitable zone depending on stellar mass and radius of planetary orbit relative to earth's orbit of the sun |
Habitable exoplanets need to orbit within a star's habitable zone, the region of space around a star where a planet would receive roughly the same energy as our Earth. This NASA site explains the formation of habitable planets.
The graphic to the left illustrates the variation in habitable zone
around different star types. The graphic shows how both the distance of
the habitable zone from the star and its width are directly dependent
on the star's mass. The width of the habitable zone can also be affected
by assumptions, such as cloud cover.
Planets substantially larger than earth could harbor life. In fact, according to one article, such super earths - from about 2 to 10 Earth masses - may be superior at fostering life.
The reader is cautioned that the definition of habitable zone
differs among sources. For example, at the beginning of 2020, three
sources presented widely different lists of planets in their respective
habitable zone. These were (1) Habitable Exoplanets Catalog [55], (2) Planetary Biology [73], and (3) Habitable Zone Gallery [131]. NASA does not list potential habitable exoplanets.
The three lists emphasize different qualities of the subject planets and their respective stars. According to Habitable Exoplanets Catalog, sixteen (16) habitable zone planets were closest to earth (within 50 light years). According to Planetary Biology, forty (40) of its listed habitable planets orbited 'G' class suns similar to earth.
An apparent disadvantage of super-earths, despite the expanded real estate, is the heavy gravity. All that additional planetary mass must be crushing.
Escaping the planet will be difficult, but super-earth gravity does not necessarily squash earthlike life forms. A remedy can be found in the planetary radius. A planet's surface gravity is determined by its mass divided by the radius squared [Gravity = M/R2]. Using that formula, a planet 9 times Earth's mass with a radius 3 times that of the Earth would have a surface gravity equal to Earth. On such a hypothetical super-earth, people would weigh the same as on Earth and have nine times the planetary surface to explore [Surface Area = 4 π x R2].
But wait, another problem now arises; the planet's density [Density = Mass/Volume = M/1.33πR3]. The density of water is 1 g/cm3. The planet Earth is 5.5 g/cm3. Our hypothetical exoplanet would have a density of 1.89 grams per cubic centimeter - closer to that of water than that of our Earth.
This density is virtually the same as Saturn's moon Titan. Titan is composed
primarily of water. A super-earth of Titan's density and composition
(located in the habitable zone with water in a liquid state) would
likely be covered with an ocean well over a thousand kilometers deep.
People may well explore its surface, but they will never find land upon
which to settle.
Mars has a density (3.93 g/cm3) a little over 71% of Earth's. It also has an iron core which at super-earth size could generate a protective magnetic belt. Using Mars as our density model rather than Titan, we find a more satisfactory exoplanet. A hypothetical super-earth, with about 2.5 times Earth's mass and a radius about 1.54 times that planet, will have a density comparable to Mars. The surface area is almost 2.5 times that of Earth. At the same time, the surface gravity will be comparable to Earth.
Other super-earth options which achieve this earth surface gravity goal are possible. I will leave the calculations to others.
Certain eccentric planets whose orbit extends outside the habitable
zone may nevertheless be habitable. While Earth and the other planets in
our solar system travel around the sun in near-circular orbits,
planets in other systems can have more comet-like orbits in which the
distance from the planet to star varies. Such orbits, termed eccentric,
would cause the planet to move in and out of the habitable zone.
The eccentric planet would move through the habitable zone and then further out into a long, cold winter. During this phase of the orbit, any liquid water on the planet will freeze at the surface; however, the possibility remains that life could, in theory, hibernate beneath the surface. These planets in eccentric orbits could still retain lifebearing properties depending upon the percentage of the total orbit which is spent within the Habitable Zone. This paper (a PDF is downloadable) describes the development of the habitable zone concept and its potential application to planets in extreme eccentric orbits. Another source discusses the influence of eccentricity on habitability.
Habitability for life similar to that found on Earth requires more than just being in the habitable zone. To be habitable, the planet where life might exist should be of the terrestrial type. A terrestrial planet is primarily composed of silicate rocks and is not a gas giant. By the beginning of 2020, NASA counted 161 terrestrial planets orbiting other stars.
In contrast, NASA says scientists have discovered 1302 gas giants. Hot Jupiters, are defined as gas planets orbiting extremely close (orbit of less than 10 days) to their parent star. Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets.
The Habitable Exoplanets Catalog [Click here to see most recent catalog] shows potential habitable exoplanets of the terrestrial type. They are compared with Earth and Mars using the Earth Similarity Index.
This index number is a measure of Earth-likeness, where Earth is the standard of comparison with an ESI value equal to one. Exoplanets with values above 0.8 could be considered Earth-like planets but those with values down to about 0.7 might still be habitable by microbial life.
The potential habitable exoplanets bear little resemblance to Earth. Most of the habitable planets circling dwarf stars are probably tidally locked with that star. The crushing gravity of some of the super-earths compared to Earth would result in the evolution of a higher level life form quite different from what we are familiar. The likelihood of a totally earth-like environment on any of the planets discovered so far is slim.
One matter of controversy respecting the existence of habitable terrestrial planets is the need for a large moon (such as Earth's moon) to stabilize the planet's obliquity (or axial tilt). An unstable obliquity could produce severe climate changes, potentially affecting the development of life. Many scientists theorized that the need for such a moon, and its expected rarity, made habitable planets less likely. A recent study concluded that the need for a moon to achieve such stability is over blown. This video of a SETI Talk presentation by Jack Lissauer goes into more detail regarding the newest thinking.
The three lists emphasize different qualities of the subject planets and their respective stars. According to Habitable Exoplanets Catalog, sixteen (16) habitable zone planets were closest to earth (within 50 light years). According to Planetary Biology, forty (40) of its listed habitable planets orbited 'G' class suns similar to earth.
Super Earths; Gravity, Radius and Density
An apparent disadvantage of super-earths, despite the expanded real estate, is the heavy gravity. All that additional planetary mass must be crushing.
Escaping the planet will be difficult, but super-earth gravity does not necessarily squash earthlike life forms. A remedy can be found in the planetary radius. A planet's surface gravity is determined by its mass divided by the radius squared [Gravity = M/R2]. Using that formula, a planet 9 times Earth's mass with a radius 3 times that of the Earth would have a surface gravity equal to Earth. On such a hypothetical super-earth, people would weigh the same as on Earth and have nine times the planetary surface to explore [Surface Area = 4 π x R2].
But wait, another problem now arises; the planet's density [Density = Mass/Volume = M/1.33πR3]. The density of water is 1 g/cm3. The planet Earth is 5.5 g/cm3. Our hypothetical exoplanet would have a density of 1.89 grams per cubic centimeter - closer to that of water than that of our Earth.
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| Internal structure of Titan, largest moon of Saturn, showing its ice layers Courtesy NASA |
Mars has a density (3.93 g/cm3) a little over 71% of Earth's. It also has an iron core which at super-earth size could generate a protective magnetic belt. Using Mars as our density model rather than Titan, we find a more satisfactory exoplanet. A hypothetical super-earth, with about 2.5 times Earth's mass and a radius about 1.54 times that planet, will have a density comparable to Mars. The surface area is almost 2.5 times that of Earth. At the same time, the surface gravity will be comparable to Earth.
Other super-earth options which achieve this earth surface gravity goal are possible. I will leave the calculations to others.
Habitable Zone and Eccentrics
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| Eccentric exoplanet passing through and outside its star's habitable zone, Courtesy NASA-JPL Caltech |
The eccentric planet would move through the habitable zone and then further out into a long, cold winter. During this phase of the orbit, any liquid water on the planet will freeze at the surface; however, the possibility remains that life could, in theory, hibernate beneath the surface. These planets in eccentric orbits could still retain lifebearing properties depending upon the percentage of the total orbit which is spent within the Habitable Zone. This paper (a PDF is downloadable) describes the development of the habitable zone concept and its potential application to planets in extreme eccentric orbits. Another source discusses the influence of eccentricity on habitability.
Terrestrial Type
Habitability for life similar to that found on Earth requires more than just being in the habitable zone. To be habitable, the planet where life might exist should be of the terrestrial type. A terrestrial planet is primarily composed of silicate rocks and is not a gas giant. By the beginning of 2020, NASA counted 161 terrestrial planets orbiting other stars.
In contrast, NASA says scientists have discovered 1302 gas giants. Hot Jupiters, are defined as gas planets orbiting extremely close (orbit of less than 10 days) to their parent star. Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets.
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| These 21 exoplanets listed in 2020 are more likely to have a rocky composition and maintain surface liquid water (i.e. 0.5 < Planet Radius ≤ 1.5 Earth radii or 0.1 < Planet Minimum Mass ≤ 5 Earth masses). Planetary Habitability Laboratory, University of Puerto Rico at Arecibo |
This index number is a measure of Earth-likeness, where Earth is the standard of comparison with an ESI value equal to one. Exoplanets with values above 0.8 could be considered Earth-like planets but those with values down to about 0.7 might still be habitable by microbial life.
The potential habitable exoplanets bear little resemblance to Earth. Most of the habitable planets circling dwarf stars are probably tidally locked with that star. The crushing gravity of some of the super-earths compared to Earth would result in the evolution of a higher level life form quite different from what we are familiar. The likelihood of a totally earth-like environment on any of the planets discovered so far is slim.
Need for a Moon?
One matter of controversy respecting the existence of habitable terrestrial planets is the need for a large moon (such as Earth's moon) to stabilize the planet's obliquity (or axial tilt). An unstable obliquity could produce severe climate changes, potentially affecting the development of life. Many scientists theorized that the need for such a moon, and its expected rarity, made habitable planets less likely. A recent study concluded that the need for a moon to achieve such stability is over blown. This video of a SETI Talk presentation by Jack Lissauer goes into more detail regarding the newest thinking.
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