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42

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 magnetosphere. As such, I'm going to interpret your question as, can a planet without a magnetosphere retain an atmosphere long-term? As proof, I offer up Venus....


41

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 measurement — this detects the small movement of the star towards and away from us as the planet and the star orbit their mutual barycenter. This movement is fastest ...


38

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" then its gravity won't hold on to any water vapour, and it will be lost to space. The Earth can have liquid water because its gravity is strong enough to ...


30

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 reflected light. Note that the top of the line also curves to reflect how much of the planet's surface is illuminated from the star. The most illumination is ...


26

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 planet. Of these, only one fits the criteria of being Earth-sized and well placed in the habitable zone: Kepler 186-f However we know nothing about it's ...


15

In 2013, the smallest detected exoplanet was Kepler-37-b, which is actually smaller in mass and size than Earth, so we already had a limited ability to detect these size exoplanets. It is worth noting that there wasn't any new technology that allowed this advance, the paper linked to by NASA indicates that the same methods were used as they would usually ...


15

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 massive, therefore sufficiently large. The warmer a planet, the more mass it needs to sustain an atmosphere. A planet warm enough for liquid water must thus also ...


14

This is a very interesting question. Of course, as you noted, you have simplified things quite a bit; there are other factors besides temperature that affect habitability. Regarding Venus, you probably know that Venus is extremely hot at its surface not just because it is closer to the Sun, but because it has a thick CO2 atmosphere and is warmed by the ...


13

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 observationally sensible as radius is directly deducable from transits (assuming stellar size known) and mass is often an accessible properties for transiting planets ...


12

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 you may know, axial tilt affects the seasons because the light is "spread out" when it falls on an angled surface. Note that tilt causes different seasons in ...


12

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 start at the very beginning (a very good place to start). The very early years When our solar system began forming 4.6 billion years ago, most of the mass ...


11

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 of 8.43 on a logarithmic (base 10) scale where hydrogen has an abundance of 12 (Asplund et al. 2009). This is typical for stars in the solar neighbourhood. ...


11

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 atmospheric conditions, to possibly permit the existence of liquid water on the surface. That's not a surefire sign that the planet is capable of supporting ...


10

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 and brown dwarfs What you need is a mass-luminosity relation combined with an expression for the tidal radius in terms of the stellar mass. The latter also ...


9

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 $$r_P=\frac{\rho_E}{\rho_P}r_E,$$ with $\rho_E=5.515 \mbox{ g}/\mbox{cm}^3$ the mean density of Earth, $r_E=6371.0 \mbox{ km}$ the mean radius of Earth, $\rho_P=22.59\mbox{ g}/\mbox{...


9

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 or so new exoplanets, we've only gone so far as to show that they (statistically) exist and to measure some very very basic properties of them. It is on these ...


9

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 (drastically different) forms? and I'm always bothered by these reports because they are all predicated on the premise that life on other planets will closely resemble ...


9

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 gravity to keep water in the atmosphere. However, I wanted to mention this: However, the conditions required for liquid water can be extended by mixing it with other ...


8

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 humans. We barely have the technology to detect some planets orbiting a small number of other stars (compared to the number of stars in the galaxy). The term for ...


8

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 ($\frac1{0.723332}^2\approx1.9113$). Even when the solar system was young and the Sun was considerably dimmer than it is now, Venus received 1/3 more solar radiation at the top of ...


7

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 find some metamorphic remnants which have survived the last resurfacing, and one could determine the age of rocks. Might be there exist some old layers below the ...


6

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". The initial searches for narrow-band radio emission (at 1.1-1.9 Hz) appears to have concentrated on those systems containing multiple planets. The (null) ...


6

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 hot all over the planet a part of the year, and than cold all over the planet the other part of the year. On earth, due to the axial tilt, we have winter in the ...


6

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 orbit's eccentricity: a very eccentric orbit would cause a whole planet "summer" when around the perihelion and a whole planet "winter" near the aphelion. There ...


6

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 much less of Venus' atmosphere, even in comparison to the amount Earth lost, as Earth is considerably closer to the Sun than Mars. So, Venus would perhaps have ...


6

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 as the Sun approaches the end of its life. As a result, the orbital radii of the planets would increase. The surface temperature of this future AGB Sun would be ...


6

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 about the distance of the moon would take around a month. If the object really had negligible mass it would be moving a little slower than the moon, so let's be ...


6

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 documents get periodicaly published, so keep an eye on the page pointed out by @planetmaker on the comments. The robot has measured a total of 1759 Marsquake events as ...


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