Links to the other pages:

• Main page
• Which primary conditions are necessary for life as we know it?
• What is the difference between a planet and a star and can there be life on these objects?
• How does the 'global' structure of a solar-system contribute to the possibility of creation and development of life as we know it?
• Which classes of planets can we detect with current observational techniques?
• How do stellar populations vary at different places within our Galaxy?

This page:

• introduction
• habitable zone
• discovered planets with data
• conclusion
• links
• articles

The question can be divided in some other subquestions:

• Which conditions are necessary for a planet to support life?

• Which planets have been detected so far that match these conditions?

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)

,

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 habitable 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.

* From: Habitable Zones around Main Sequence Stars; Kasting, J. F.; Whitmire, D. P.; Reynolds, R. T. ³

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

Since all planets are probably gaseous, you wouldn't expect life to evolve there.

table 1: Some data of planets around main sequence stars (data from Darwin Project and Extra-solar Planets Catalog) :

As you might notice, the mass is given in Jupiter mass·sin i, where i
is the inclination of the planet's orbit. Because the orbital inclination
cannot be retreived from the observations, the mass range could be
quite big.
For the temperature estimate is used: Albedo = 0.5, Rs = Rsun (~ 4.74 .10^-3 AU)
then Tp = (0.041/a^1/2).Ts, a in AU

table 2: Some data on Earth and Jupiter and the Sun:
 

Planet	Mass  10^24 kg	Mass  MJupiter	Period  days	Orbit  AU	Eccentricity	Radius  Earth (6,378 km)	Albedo	Temperature  Kelvin
Earth	5.97	0.0031	365.26	1	0.017	1	0.39	250-300
Jupiter	1900	1	4333	5.203	0.048	11.19	0.51	110-150
Star	Spectral  Type	Temperature  Kelvin	Mass  kg	Radius  km
Sun	G2V	5770	1.99 .10^30	6.96 .10^5

figure 1: Some of the discovered planets (MJup is actually MJup .sin i ):
 

 

Conclusion

The planets that have been detected so far around main sequence stars, are probably all gaseous, like Jupiter. Planets around main sequence stars are currently the only planets where we would expect life to evolve.
Finding planets like Earth could take a while. Our detection precision has to be bettered a great deal to find such planets around main sequence stars like our sun. But maybe there will be found some clear useful data in the near future, that'll give some hopeful results for the possibility of life on extra-solar planets.
Untill then the observations will continue and more planets will be discovered as time passes by.

Who knows what the future might bring....

Links:

Darwin Project Planets outside the Solar System found and searched for.

Extra-solar Planets Catalog An overview of detected extra-solar planets.

Exoplanets Overview of detected planets and links to other sites related to this subject.

Astrobiology Some things related to life in the Universe and other interesting things.

 Interesting Articles:

¹Title: Strategies for the search of life in the universe
Author: Schneider, J
-1996

²Title: The Lick Observatory Planet Search
Authors: Butler, R. P.; Marcy, G. W.
-1996

³Title: Habitable Zones around Main Sequence Stars
Authors: Kasting, J. F.; Whitmire, D. P.; Reynolds, R. T.
Journal: Icarus, 101, 108-128 (1993)

Title: Searching for life on other planets.
Authors: ROGER, J. A. P.; WOOLF, N. J.
Journal: Scientific American, 274, part no 4, 46-52 (1996)
Bibliographic Code: 1996SciAm.274d..46R
Abstract: The recent thrilling discoveries of planets around other stars are only the beginning. If astronomers are to learn whether there are worlds like our own, they will need new types of telescopes that can identify the telltale elemental signatures of life despite light-years of distance and the glare of other suns.

Title: Life in the Universe.
Authors: Special issue of Scientific American.
Journal: Scientific American, 271, part no 4 (10/1994)
Abstract: Special Issue of Scientific American about life in the universe. top

© M. Kleefman, 1997

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