Introduction
Does life exist on other planets beyond our Solarsystem? There is a high probability that life does exist on other planets than Earth.
But what do mean by 'life'? When we're talking about life we mean life as we know it: carbon based organic life forms that needs liquid water to exist.
So not all planets are capable of sustaining life. We know that already for quite some time, because in our Solarsytem Earth is the only planet of which we certain know that it sustains life. Mars could also have had life on it, but that isn't for sure.
A planet has to meet certain conditions to be able to support life; the main condition is that the planet has to lie in the habitable zone. This is the region around a star in which life-supporting planets can exist; the boundaries are named the inner and outer edge of the habitable zone.
This means the habitable zone of a star requires certain conditions for a planet:
1. the star has to be a main sequence star (i.e. a star burning steadily light elements into heavy ones)
2. the planet has to be solid to allow for a liquid-solid interface, this to enhance the exchange between molecules
3. the planet has to be at the right distance from the star to allow for liquid water (temperature dependence)
With the formula below (J. Schneider)¹ we can calculate the equilibrium temperature of a planet orbiting a certain star. A planet acquires, by heating, an equilibrium temperature Tp given by:
Where A is the mean albedo (reflectance) of the planet surface at a distance a around a star with radius Rs and temperature Ts.
On this page I will outline some things related to the question on top of this page.
Habitable zone * (see also Bjorn's and Saskia's page)
The habitable zone (HZ) is the region around a star in which life-supporting planets can exist (Huang 1959,1960).
The habitable zone for Earth-like planets orbiting main sequence stars, is determined by water loss on the inner edge and by CO2 condensation, leading to runaway glaciation, on the outer edge. Planetary habitability is critically dependent on atmospheric CO2 and its control by the carbonate-silicate cycle. Conservative estimates for the boundaries of the Sun's (G type star) current HZ are 0.95 AU for the inner edge and 1.37 AU for the outer edge. The actual HZ width is probably greater, but is difficult to determine an exact value because of uncertainties regarding clouds which affect the planetary albedo.
HZ widths around other stars in the spectral classification range of interest, F to M (~7200 to ~3000 Kelvin), are approximately the same if distances are expressed on a logarithmic scale (i.e. if you plot the distances from the inner and outer edges of the CHZs for different stars on a logarithmic axis, you will find that the widths of the CHZs for the different stars is about the same on this scale). If planets exist around other stars (they do) and if planetary spacing is logarithmic, as in our Solar System, the chances that one or more planets will be found within a star's HZ are fairly good.
The continuously habitable zone (CHZ) is the HZ that stays the HZ during the lifetime of the star. Because the star evolves the boundaries of the HZ will change slightly too, the CHZ will not change so the width of the CHZ will be smaller than the width of the HZ.
The width of the continuously habitable zone (CHZ) around a star depends on the time that a planet is required to remain ha
bitable and on whether a planet that is initially frozen can be cold-started by a modest increase in stellar luminosity. CHZs are generally narrower than HZs because the boundaries of the CHZ migrate outward as a star ages. Despite this, the 4.6 Gyr CHZ around our own Sun extends from at least 0.95 to 1.15 AU and is probably considerably wider.
CHZs around early K stars should be somewhat wider (in log distance) than around G stars because the K stars evolve more slowly. Equivalently, one could say that their CHZs are longer-lived. Since there are approximately three times as many K stars as G stars, this suggests that the majority of habitable planets may reside around K stars. Late K stars and M stars would have even wider CHZs, but the planets within them are susceptible to tidal damping and will probably rotate synchronously after a few billion years. F stars should have narrower CHZs than do G stars (on a log distance scale) because they evolve more rapidly. High ultraviolet flux are another potential problem for life around F stars. Stars earlier than ~F0 have main sequence lifetimes of less than 2 Gyr, so their planets are probably not suitable for evolving intelligent life. But 'simple life' could evolve here.
Discovered planets
Most extra-solar planets that have been discovered have been found by using Doppler technique. I've listed a table and a schematic diagram of the recently discovered planets around main sequence stars. I've also made a table with some data on Jupiter and Earth. These are planets who are the most likely to support life. As you can see most planets have small orbital values, and the mass is also quite big. Solid planets most have masses of ~15 Earthmasses, planets with higher mass are mostly of the gaseous type.
This means the surface temperature would be way to high to support life, and the planets would all be of the gaseous type. None of them is likely to be solid and none of them is likely to be a candidate for a life sustaining planet.
There have also been planets found orbiting pulsars. A pulsars is a radio source that emits signals in very short, regular bursts; it's a highly magnetic, rotating star of extremely high density and small size that is composed mainly of very tightly neutrons (neutron star, mass no bigger than ~3 solar masses). We expect here more extreme conditions, and a habitable zone is not very likely.
The object orbiting these pulsars are most Earth like masses and solid, but there have also Jupiter like masses been found; data can be found at Darwin Project and Extra-solar Planets Catalog.
Objects with mass > 13 Jupiter masses are commonly named Brown Dwarfs. This is a very low mass objects (~0.01-0.08 solar mass) of low temperature and luminosity that never becomes hot enough in its core to ignite thermonuclear reactions. So you can't really call them planets, they are some kind of stars that have failed to become a star. Several of these kind of object have also been found orbiting stars; data can be found at Darwin Project and Extra-solar Planets Catalog.
But why have only these kind of planets been found orbiting main sequence stars? The answer lies in the Doppler technique used to find these planets. These kind of planets are easiest to discern using this observation technique. To discover less massive planets in more high orbit you would need more high-precision Doppler observations, but that isn't conceivable yet.
You could also use more precise observation techniques like micro-lensing, but micro-lensing events are more rare and there's only one chance to collect the data.
Let's make some assumptions for the quantities in the formula above to estimate the planets surface temperature.
* The stars listed in the table are all of the F and G type. This means the temperatures of the stars ranges from ~5100 to ~7200 degrees Kelvin. Use this for Ts.
* The mean albedo A for earth is 0.39, that of Jupiter is 0.51. Use a value of the same size here also.
* The radius Rs of typeV G and F stars is about the same as the radius of the sun, 6.96 .10^5 km (range is about 1.3 Rsun (F0V) to 0.85 Rsun (G9V)).
* The value for a is given in the table below, the radius of the orbit.
* 1 AU = 1.496 .10^8km
The estimated temperatures of the stars and the calculated temperatures of the planets are listed in table 1.
One can see as the orbit becomes bigger the temperature drops. Some of the planets have high eccentricity's, this means that the temperature will vary a lot, because of the smaller and greater distance from the star.
Since all planets are probably gaseous, you wouldn't expect life to evolve there.
Does life exist on other planets beyond our Solarsystem? There is a high probability that life does exist on other planets than Earth.
But what do mean by 'life'? When we're talking about life we mean life as we know it: carbon based organic life forms that needs liquid water to exist.
So not all planets are capable of sustaining life. We know that already for quite some time, because in our Solarsytem Earth is the only planet of which we certain know that it sustains life. Mars could also have had life on it, but that isn't for sure.
A planet has to meet certain conditions to be able to support life; the main condition is that the planet has to lie in the habitable zone. This is the region around a star in which life-supporting planets can exist; the boundaries are named the inner and outer edge of the habitable zone.
This means the habitable zone of a star requires certain conditions for a planet:
1. the star has to be a main sequence star (i.e. a star burning steadily light elements into heavy ones)
2. the planet has to be solid to allow for a liquid-solid interface, this to enhance the exchange between molecules
3. the planet has to be at the right distance from the star to allow for liquid water (temperature dependence)
With the formula below (J. Schneider)¹ we can calculate the equilibrium temperature of a planet orbiting a certain star. A planet acquires, by heating, an equilibrium temperature Tp given by:
Where A is the mean albedo (reflectance) of the planet surface at a distance a around a star with radius Rs and temperature Ts.
On this page I will outline some things related to the question on top of this page.
Habitable zone * (see also Bjorn's and Saskia's page)
The habitable zone (HZ) is the region around a star in which life-supporting planets can exist (Huang 1959,1960).
The habitable zone for Earth-like planets orbiting main sequence stars, is determined by water loss on the inner edge and by CO2 condensation, leading to runaway glaciation, on the outer edge. Planetary habitability is critically dependent on atmospheric CO2 and its control by the carbonate-silicate cycle. Conservative estimates for the boundaries of the Sun's (G type star) current HZ are 0.95 AU for the inner edge and 1.37 AU for the outer edge. The actual HZ width is probably greater, but is difficult to determine an exact value because of uncertainties regarding clouds which affect the planetary albedo.
HZ widths around other stars in the spectral classification range of interest, F to M (~7200 to ~3000 Kelvin), are approximately the same if distances are expressed on a logarithmic scale (i.e. if you plot the distances from the inner and outer edges of the CHZs for different stars on a logarithmic axis, you will find that the widths of the CHZs for the different stars is about the same on this scale). If planets exist around other stars (they do) and if planetary spacing is logarithmic, as in our Solar System, the chances that one or more planets will be found within a star's HZ are fairly good.
The continuously habitable zone (CHZ) is the HZ that stays the HZ during the lifetime of the star. Because the star evolves the boundaries of the HZ will change slightly too, the CHZ will not change so the width of the CHZ will be smaller than the width of the HZ.
The width of the continuously habitable zone (CHZ) around a star depends on the time that a planet is required to remain ha
bitable and on whether a planet that is initially frozen can be cold-started by a modest increase in stellar luminosity. CHZs are generally narrower than HZs because the boundaries of the CHZ migrate outward as a star ages. Despite this, the 4.6 Gyr CHZ around our own Sun extends from at least 0.95 to 1.15 AU and is probably considerably wider.CHZs around early K stars should be somewhat wider (in log distance) than around G stars because the K stars evolve more slowly. Equivalently, one could say that their CHZs are longer-lived. Since there are approximately three times as many K stars as G stars, this suggests that the majority of habitable planets may reside around K stars. Late K stars and M stars would have even wider CHZs, but the planets within them are susceptible to tidal damping and will probably rotate synchronously after a few billion years. F stars should have narrower CHZs than do G stars (on a log distance scale) because they evolve more rapidly. High ultraviolet flux are another potential problem for life around F stars. Stars earlier than ~F0 have main sequence lifetimes of less than 2 Gyr, so their planets are probably not suitable for evolving intelligent life. But 'simple life' could evolve here.
Discovered planets
Most extra-solar planets that have been discovered have been found by using Doppler technique. I've listed a table and a schematic diagram of the recently discovered planets around main sequence stars. I've also made a table with some data on Jupiter and Earth. These are planets who are the most likely to support life. As you can see most planets have small orbital values, and the mass is also quite big. Solid planets most have masses of ~15 Earthmasses, planets with higher mass are mostly of the gaseous type.
This means the surface temperature would be way to high to support life, and the planets would all be of the gaseous type. None of them is likely to be solid and none of them is likely to be a candidate for a life sustaining planet.
There have also been planets found orbiting pulsars. A pulsars is a radio source that emits signals in very short, regular bursts; it's a highly magnetic, rotating star of extremely high density and small size that is composed mainly of very tightly neutrons (neutron star, mass no bigger than ~3 solar masses). We expect here more extreme conditions, and a habitable zone is not very likely.
The object orbiting these pulsars are most Earth like masses and solid, but there have also Jupiter like masses been found; data can be found at Darwin Project and Extra-solar Planets Catalog.
Objects with mass > 13 Jupiter masses are commonly named Brown Dwarfs. This is a very low mass objects (~0.01-0.08 solar mass) of low temperature and luminosity that never becomes hot enough in its core to ignite thermonuclear reactions. So you can't really call them planets, they are some kind of stars that have failed to become a star. Several of these kind of object have also been found orbiting stars; data can be found at Darwin Project and Extra-solar Planets Catalog.
But why have only these kind of planets been found orbiting main sequence stars? The answer lies in the Doppler technique used to find these planets. These kind of planets are easiest to discern using this observation technique. To discover less massive planets in more high orbit you would need more high-precision Doppler observations, but that isn't conceivable yet.
You could also use more precise observation techniques like micro-lensing, but micro-lensing events are more rare and there's only one chance to collect the data.
Let's make some assumptions for the quantities in the formula above to estimate the planets surface temperature.
* The stars listed in the table are all of the F and G type. This means the temperatures of the stars ranges from ~5100 to ~7200 degrees Kelvin. Use this for Ts.
* The mean albedo A for earth is 0.39, that of Jupiter is 0.51. Use a value of the same size here also.
* The radius Rs of typeV G and F stars is about the same as the radius of the sun, 6.96 .10^5 km (range is about 1.3 Rsun (F0V) to 0.85 Rsun (G9V)).
* The value for a is given in the table below, the radius of the orbit.
* 1 AU = 1.496 .10^8km
The estimated temperatures of the stars and the calculated temperatures of the planets are listed in table 1.
One can see as the orbit becomes bigger the temperature drops. Some of the planets have high eccentricity's, this means that the temperature will vary a lot, because of the smaller and greater distance from the star.
Since all planets are probably gaseous, you wouldn't expect life to evolve there.
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