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TimberWolf
2004-Jul-30, 03:18 PM
Hello,

I was curious about how astronomers could determine the habitability zones of different stars. Considering that distance from the parent star, spectral class of the parent and the orbital eccentricity of such prospective worlds would be major factors in such an equation. I had read in an astronomy text by Isaac Asimov that only dim Fs, Gs and Ks would be capable of hosting planets anywhere near Earth-like, so I doubt Rigel, Betelgeuse or Ross 154 could have a habitable planet orbiting them.

Cordially,

TimberWolf

gritmonger
2004-Jul-30, 06:03 PM
Far as I know, "habitable" does not equal "capable of evolving life or intelligent life" It has more to do with the degree of insolation, mean surface temperature, possibility for liquid water and the like. You could have a habitable but barren world where you need oxygen or an environmental suit to go out of doors, but little in the way of temperature assist vs. very hot or very cold.

The habitable zones around hot, large stars are much more expansive than those around M, L or T class ( if T even has one... ) - but since the stars don't stick around near as long, as well as putting out a lot more radiation in the UV and up in the case of white and blue stars, you don't expect them based on our experience of one planet to be capable of evolving intelligent life, or even life in the case of blue giants.

[edited for clarity]

Brady Yoon
2004-Jul-30, 06:19 PM
Planets around stars F-K have the most chance of life forming (based on current knowldege) , because they have the right balance of time for life to form and a sufficiently large habitable zone.

Kullat Nunu
2004-Jul-31, 05:25 PM
Planets around stars F-K have the most chance of life forming (based on current knowldege) , because they have the right balance of time for life to form and a sufficiently large habitable zone.

True, but if stellar flares, tidal lock (planet always turning the other side to the star), and dearth of energy do not make life impossible, vast numbers of red dwarfs could have life-harboring planets.

Brady Yoon
2004-Jul-31, 10:31 PM
True, but if stellar flares, tidal lock (planet always turning the other side to the star), and dearth of energy do not make life impossible, vast numbers of red dwarfs could have life-harboring planets

Yeah, life would literally have trillions of years to form.

TimberWolf
2004-Aug-01, 01:43 AM
Hello,

Good posts everyone, but still my question remains: How do astronomers determine the habitability zones of stars? I remember reading an astronomy article a couple years of ago on how they could but, unfortunately, I don't remember the method. From what I gather, the conditions of the parent star are critical. Such as the energy output of the star, age, stability and exactly how all the different gravity wells interact in the planetary system, including the distance of the planet from the parent and orbital eccentricity.

I don't mean to be rude, sorry if I was.

Cordially,

TimberWolf

Brady Yoon
2004-Aug-01, 03:06 AM
Good posts everyone, but still my question remains: How do astronomers determine the habitability zones of stars?

Astronomers determine habitable zones by determining where in space the freezing and boiling points of water would be. The habitable zone is essentially the zone between those two areas. Keep in mind that the habitable zone is a 3 dimensional area in space.


From what I gather, the conditions of the parent star are critical. Such as the energy output of the star, age, stability and exactly how all the different gravity wells interact in the planetary system, including the distance of the planet from the parent and orbital eccentricity

If I'm not mistaken, the limit and size of the habitable zone is only dependent on the luminosity of the star.

Brady Yoon
2004-Aug-01, 03:12 AM
For clarification, the habitable zones are determined by the temperatures caused by direct radiation. It is assuming that the amount of radiation emitted by the sun is balanced by the amount absorbed by the planet. This is not the case when it comes to Earth. Because of the atmosphere's greenhouse effect, Earth absorbs more radiation than emitted. If it weren't for the greenhouse effect, Earth's average temperature would be around 0 degrees Celsius. It is on the outside border of the solar system's habitable zone. Habitable zones only are applicable when an objects is a black body, which is a perfect emitter and absorber. It's assuming that the planet has no atmosphere.

stu
2004-Aug-01, 03:26 AM
A paper on the subject that I've used in the past is Kasting et al.'s "Habitable Zones Around Main Sequence Stars" that can be found at http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993Icar..101..108K&db_key=A ST&high=407c2049a621827 .[/url]

sol_g2v
2004-Aug-01, 08:44 AM
Here (http://www.newscientist.com/hottopics/astrobiology/redwilling.jsp) is an essay by Ken Croswell describing the habitable zone around red dwarfs.

TimberWolf
2004-Aug-02, 03:34 AM
Hello,

Cool. Thanks,everyone. From what I gather, NASA is planning to hunt for Earth-size planets with the TPF. How do they plan on doing that? I heard that the technique involved is the use of some sort of infrared or thermal spectronomy.

Cordially,

TimberWolf

eburacum45
2004-Aug-02, 09:38 PM
I might just take the opportunity to suggest that some estimates of habitable zones are wrong; if it is estimated by comparing the visual brightness of the star, this will not be accurate for reddish stars or blueish stars, but only for sun-like yellow stars.

If you try to put a habitable planet around a reddish star so that it is in the 'visual comfort zone' where the star is roughly the same visual brightness as our Sun as seen from Earth, you will find that the infrared component of the stars radiation will heat the planet too much.
Similarly the ultraviolet component of blueish stars will heat a planet in the visible CZ. In actual fact many blue stars will have no real habitable zone at all because of UV.

What is needed is a measure of the heat energy falling on a planet, visible or not; the Bolometric luminosity is a measure of that. So a planet in the real, bolometric habitable zone, will be further out from the star in every case except that of very yellow, sun-like stars.

Another consequence is that yellow stars have the greatest apparent brightness in the habitable zone of all stars. Red dwarfs are large and dimm, but warm; bluer stars are small, and distant to allow for the invisible UV energy so appear less bright in the visible spectrum.

Brady Yoon
2004-Aug-02, 09:44 PM
I might just take the opportunity to suggest that some estimates of habitable zones are wrong; if it is estimated by comparing the visual brightness of the star, this will not be accurate for reddish stars or blueish stars, but only for sun-like yellow stars.

If you try to put a habitable planet around a reddish star so that it is in the 'visual comfort zone' where the star is roughly the same visual brightness as our Sun as seen from Earth, you will find that the infrared component of the stars radiation will heat the planet too much.
Similarly the ultraviolet component of blueish stars will heat a planet in the visible CZ. In actual fact many blue stars will have no real habitable zone at all because of UV.

What is needed is a measure of the heat energy falling on a planet, visible or not; the Bolometric luminosity is a measure of that. So a planet in the real, bolometric habitable zone, will be further out from the star in every case except that of very yellow, sun-like stars.

Another consequence is that yellow stars have the greatest apparent brightness in the habitable zone of all stars. Red dwarfs are large and dimm, but warm; bluer stars are small, and distant to allow for the invisible UV energy so appear less bright in the visible spectrum.

Good post. I completely missed the point about the bolometric luminosity. :)