# Tag Info

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### Can a tectonically inactive planet retain a long-term atmosphere?

Yes, a tectonically inactive planet can retain a long-term atmosphere. You make the connection that a lack of plate tectonics on a planet indicates a "dead" core and thus said planet has no ...
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### Why are most discovered exoplanets heavier than Earth?

There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large: Radial velocity ...
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### Do celestial objects need to be big to have liquid water on their surfaces?

Liquid water can't exist in a vacuum. If there is no pressure, then the boiling point will drop to the freezing point and so there will either be ice or water vapour. And if the world is "small&...
• 88.7k
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### Exoplanet dip in transit light curve when the planet passes behind the star

Just before the planet goes behind the star, we see the light directly from the star as well as the light reflected from the planet's surface. When the planet is behind the star, we no longer see the ...
• 13.8k
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### How many planets have we discovered that can support human life?

There is currently only one planet known to be capable of supporting human life, and you're on it. Several planets have been found in the region in which we expect water to be liquid on much of the ...
• 88.7k

### Do celestial objects need to be big to have liquid water on their surfaces?

Do celestial objects need to be big to have liquid water on their surfaces? Yes. In a nutshell: liquid surface water needs an atmosphere. To sustain an atmosphere, a planet must be sufficiently ...
• 1,903
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### How is an exoplanet characterised as "Earth-Like"?

There is no unique definition of Earth-like and it depends on what characteristics are important in the context of discussion. The most simple one is to just compare radii and masses. That is ...
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### Why is the Earthian atmosphere so thin?

The short answer: Atmospheric gases never left Earth, they're in it! The long answer to this question isn't just about the planets' current states, but rather the processes that led them there. Let's ...
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### Why is carbon so rare on the Moon and on Mars?

There is not much doubt that the abundance of carbon in the protosolar nebula was not abnormal. We can tell that by looking at the carbon abundance in the atmosphere of the Sun - it has an abundance ...
• 115k
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### Would an exoplanet without axial tilt have no seasons?

There are several factors that cause seasons. In approximate order from least to most speculative: Axial Tilt: This is by far the significant factor in determining what the season is on Earth. As ...
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### How many planets have we discovered that can support human life?

You may be interested in the number of habitable zone planets. These are planetary-mass objects in a sort of "Goldilocks" region from their respective stars: This region is just right, given adequate ...
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### How small a star can provide Sun-level illumination to its planets?

I consider small stars to mean main sequence stars (or brown dwarfs) with mass less than the Sun, and then also consider compact stellar remnants - white dwarfs and neutron stars. Main sequence stars ...
• 115k

### How small can a planet be and still have Earth-like gravity?

The surface gravity of a planet is very close to $$g=\frac{4\pi G}{3}\rho r.$$ With $g$ to be kept constant, and $\frac{4\pi G}{3}$ a constant, we need $\rho_Pr_P=\rho_Er_E$, or r_P=\frac{\rho_E}{\...
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### Are there any common characteristics of habitable planets?

I think I will answer your question from two vantage points. Firstly, what do we have to measure about a new exoplanet to consider it potentially habitable? Keep in mind that in detecting these 1000 ...
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### What if we are looking for the wrong signs of life on other planets?

What if we are looking for the wrong signs of life on other planets? Are there any fields of astronomy that look beyond the "near-Earth" life comparisons and look for life in other (...
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### Do celestial objects need to be big to have liquid water on their surfaces?

gerrit's answer has done an excellent job of showing that (1) there are a narrow set of temperatures and pressures where liquid water exists and (2) a planet has to be pretty big to have enough ...

### Is there another habitable planet in our galaxy?

what'll happen when we destroy our planet or it becomes none-habitual for human life? Very pessimistic idea. We lack the technology to tell if a planet orbiting another star could be lived on by ...
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### Why Venus evolved so differently from Earth?

Venus orbits the Sun at 0.723332 AU. As solar radiation follows an inverse square law, the intensity of the Sun at the top of Venus's atmosphere is almost twice that for the Earth (\$\frac1{0.723332}^...
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### Can we find out whether early Venus was Earth-like or not?

My ad-hoc opinion: This wouldn't be the first step of Venus exploration. Geologic in-situ investigation of the resurfacing hypotheses would already be a very challenging mission. Might be, one could ...
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### How should exoplanet discoveries affect SETI?

Well you are correct and of course that kind of research has been going on. The SETI programme targeted a set of Kepler stars with evidence for a planet (or planet candidate) in the "habitable zone". ...
• 115k

### Would an exoplanet without axial tilt have no seasons?

If it would not have any axial tilt, and it rotates in an orbit thats not really eccentric, it would have no seasons. If it has (really) high eccentricity, it would have seasons, but they would be ...

### Would an exoplanet without axial tilt have no seasons?

Exactly. Seasons and its difference between northern and southern hemisphere are caused due to the tilt of the Earth's axis. But this is not the only thing involved. An other important fact is the ...
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### If Venus and Mars changed places, would we then have 3 habitable planets?

If, Venus had started out at Mars' position, it is possible it would've ended up evolving into an ice giant. At the greater distance of Mars the Sun's solar wind is weaker and would've stripped off ...
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### Sun as our Supergiant

When the Sun becomes a red supergiant (more accurately, an asymptotic giant branch star), it will have less mass than it does now. That is because mass is lost by an increasingly massive stellar wind ...
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### Could a moon orbit an Earth size planet in such a way as to create a solar eclipse lasting for several weeks from start to finish?

Short answer: No, it could not. The moon already has little enough mass relative to the earth so that its orbital period is not much shorter than a satellite with negligible mass. So, any object at ...
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### How many quakes have been detected by InSight?

This is the latest catalogue from seismic events recorded by InSight: Mars Seismic Catalogue, InSight Mission; V5 2021-01-04. This report includes seismic events up to October 12, 2020. These ...
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### Help in determining the features of an unusual, fictional star system

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. ...
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### When will we be able to detect Earth-sized exoplanets?

We are now able to find smaller exoplanets than the Earth. Analysis of Kepler data has yielded the smallest known mass for an exoplanet orbiting a normal star. Its mass and size are similar to ...
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