# Tag Info

15

Just because a planet is in the "Habitable zone" doesn't mean it is habitable. A planet is said to be in the habitable zone if it is possible for liquid water to exist on the surface. A planet may be in the habitable zone, and yet not be habitable if, for instance, it's atmospheric pressure is too low (like Mars) or too high (like Venus), both of which are ...

9

From your first link, the definition is: "The circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.[1][2][3][4][5] The bounds of the CHZ are based on Earth's position in the Solar System and the amount of ...

9

Nobody knows the limits of the Sun's habitable zone, or how broad or narrow it is. Here is a link to a list of various estimates of the inner, or outer, or both, edges of the circumstellar habitable zone of the Sun: https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#Solar_System_estimates Note that one well known estimate, that of Hart et al. in 1979,...

8

It's a pretty straight forward calculation of 3 factors. Distance from the sun, apparent size and albedo. I'm going to compare Jupiter to our full moon, since we're all familiar with that. Jupiter averages between 4.95 and 5.45 AU from the sun. That puts it, in comparison with our full moon (about 1.02 AU on average), that means individual square ...

7

It isn't the only way, Aaron. See the Wikipedia Methods of detecting exoplanets article. The first method listed is radial velocity. That's where we measure the Doppler shift of the star's "wobble". The second method is transit photography. And as you say it has a drawback. In fact there's more than one: "This method has two major disadvantages. First, ...

6

In the real universe, stars that go supernova are very large stars that are short lived, or, they could be white dwarfs that accrue matter and overcome the Chandrasekhar limit. Very large stars, 8 solar masses or more - about the minimum size for a star that goes supernova, the life span of that star is too short for a planet to form, cool and develop life....

5

The Kepler space telescope had a field of view along the so called Orion arm, or spur, of the galaxy. The same structure which we ourselves are inside. It basically looks along a line where the star density is the greatest within the distance that it could detect transiting planets. Less than 3% of the galactic diameter, I think. Note that the illustration ...

5

First it will be important to consider the term 'relativistic speed'. If by that you mean something like 0.1c, it will only change the colour of the stars as you mentioned in the bounty description. However, if it means something with higher Lorentz Gammas (like 0.9c or 0.99c), all other relativistic effects come into play. There's relativistic beaming and ...

5

For those who haven't seen it: The basic answer is that a planet can orbit a black hole. There are stable circum-black-hole orbits, just as there are stable orbits around just about any celestial body. There's a problem: A black hole typically forms as a result of a supernova. This will eject most nearby planets out of the stellar system. Alternatively, it'...

5

It's actually the opposite. Venus, Mars and I'll include the Earth as well, are likely getting more solar energy per square meter, not less, over time, even as they slowly move away from the sun. 1.5 cm per year, from the Rob Jeffries' answer to the 2nd question you linked is too slow to make much difference. That's 1 km every 66,000 years and about 70,...

5

Yes and yes. Transit detection is only effective for that (small) fraction of planets that pass between the star and our line of sight. Most planets will go undetected by this method. Also correct. The detection of multiple transits is required to find planets. Even then the dips can be caused by other things (e.g. grazing eclipse binary star systems). This ...

5

Habitable zones, defined in terms of equilibrium temperature, scale with the square root of the luminosity of the star. So whatever the habitable zone limits $[a_{inner},a_{outer}]$ are for the Sun, for another star a good starting guess is $\sqrt{L/L_\odot}$ times those limits*. For Betelgeuse the simple-minded square root model gives a time-varying ...

4

My kneejerk reaction is that your only option is to remove a chunk of mass from the outer part of the Sun. The Sun will respond (on a Kelvin-Helmholtz timescale), by contracting and becoming less luminous because the core temperature is lower in a less massive star. This will extend its main sequence lifetime, because only the central parts of the star are ...

4

Assume you have a spherical blackbody. The solar flux at the radius of the Earth is given to a good approximation by $L/4\pi d^2$, where $d = 1$ au. This is $f=1367.5$ W/m$^2$ (though note the distance between the Earth and the Sun has an average of 1 au). If it is a blackbody sphere it absorbs all radiation incident upon it. Assuming this is just the ...

4

The possibility of exploring exoplanets is so remote it is not a factor. We are interested in Earth-like planets as there is only one type of planet that we know can support life, and that is the Earth-like ones. We are interested in Earth-like planets as they may provide a habitat for alien life. The question of "does extra-terrestial life exist" is of ...

4

To your first question... from this paper, we can see that there are 3 types of atmospheric escape: Jeans Escape: Temperature and escape velocity factors determine the gasses and the amount that's lost. Using the chart provided in the comments, and moving Titan along the x-axis to around below Earth's position, we can see Titan will lose most gasses except ...

4

1st Q: It's been said that an astronomical body can keep it's atmosphere if the escape velocity is more than six times the average speed of the molecules. Gravity isn't the only factor that permits an object to retain an atmosphere. The Moon has gravity yet is virtually a vacuum, Mars has a surface pressure less than 1% of Earth and continues to lose its ...

4

From the Wikipedia article Circumstellar habitable zone, which is just another name for the Goldilocks zone: Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 astronomical units, though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this ...

4

A typical L-type brown dwarf is about 1200-2200 K in surface temperature and is about the size of Jupiter. Using the Stefan-Boltzmann law, we can deduce that the hottest brown dwarfs have a luminosity of$$\Big(\dfrac{2200}{5778}\Big)^4 \cdot \Big(\dfrac{1R_J}{1R_\odot}\Big)^2 = 0.00021224 L_\odot$$ According to Wikipedia, the dimmest apparent magnitude that ...

3

I agree with David Hammen. Hyperphysics is mostly a very good site but they dropped the ball on that page IMHO. Hope you don't mind a partially speculative answer, but here goes: Why does it matter if there are some areas of a planet with extreme temperatures, as long as there are other spots on the planet that are not extreme? It shouldn't ...

3

If you take a slowly rotating asteroid with a large basin of trapped water, some solved minerals, and may be some gases, close to the asteroid's surface, to get an environment similar to a black smoker by the heat of the sun, I wouldn't call it totally impossible. The formation of such an asteroid is close to the limits of my imagination, might be as a ...

3

I'll be honest. I looked up Galactic Habitable Zone on Wikipedia. The idea was introduced in 1986 and expanded upon in the book "Rare Earth" by Brownlee and Ward. Essentially the idea is that in the bulge there is too much activity (supernovae, etc.) and in the halo of the galaxy there is not enough heavy elements (carbon, etc.) for life to exist. This was ...

3

Of the gas giants, the number of "hot Jupiters" is about the same as the number of "hot Neptunes." A little under 9% of known gas giants are in the habitable zones. An article by John Wenz has the discoveries thus far collated into a chart: The claim, "most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up ...

3

I was curious about the figure given in Mick's answer (~9%), so I did some data-crunching of my own. I looked at the NASA Exoplanet Archive, in particular the table of confirmed planets. I was interested in four parameters: $a$, the planet's semi-major axis $M\sin i$, the planet's minimum mass $T$, the temperature of the star $R$, the radius of the star ...

3

There are many definitions for the "Goldilocks zone" or Habitable zone. If you want liquid water on the surface then that can happen for a wide variety of atmospheric pressures (totally unknown for the vast majority of exoplanets) and temperatures. In fact Mars is considered to be in the habitable zone using only this criteria. More strict criteria exist ...

3

Adding to userLTK's excellent answer, it is possible to have a triple star system where a sun-like star orbits a close binary where one member is a white dwarf and the other becomes a red giant that then triggers a supernova by mass transfer. The sun-like star can orbit suitably far away from the binary to make X-rays from the white dwarf and heating from ...

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