# ''Habitable'' planet close to a star

Imagine if you will a star comparable with our sun and a planet like earth orbiting at roughly the same distance around this star as our earth around the sun.

Would it be possible to place a planet somewhere between this ''earth analogue'' and the star/sun at such a distance as to get a planet comprised almost (if not) entirely of deserts and hot as all hell (compared to average earth temperatures,so anything above 48 degrees Celcius is fine) while still having liquid water (just not much of it and if needed not neccesarily above ground) and a breathable atmosphere.

ps. If such a world is possible could it still have plant life? (if need be subterranean in caves or something like that)

• Not likely as a habitable planet. Related: Could a habitable planet form with no major bodies of water? – Alexander Jul 17 at 17:18
• Possible useful reading is the series of questions starting here on creating the largest possible percentage of desert for a continent. – Ash Jul 17 at 17:26
• Could i perhaps get away with the planet being a desert but having large, complex and deep cave systems in wich liquid water exists? – Blue Devil Jul 24 at 13:49

You're basically talking about Venus. Or, more accurately, Venus if it had started out with a lot less water and CO2. Less water and CO2 to start with mean you never get the runaway greenhouse effect Venus has, leaving you a planet that's a lot like Earth, just dryer and hotter. Any rainfall you DID get would be the higher latitudes and that's where you could have plant life, although it'd be evolved for very arid conditions, just like the plants in the American Southwest.

EDIT: If you want this planet to be habitable, you also need it to have a working magnetic field, which Venus does not due to its slow rotation and/or internal composition.

• This suddenly sounds a lot easier to attain and explain than i though it was gonna be. – Blue Devil Jul 17 at 17:14
• Just understand that no matter what you're going to have SOME climatic variation just based on latitude, if nothing else. Temperatures will be cooler as you travel towards the poles, you can't avoid that, but as long as you're not trying to postulate a uniform desert planetwide, you're ok. – Morris The Cat Jul 17 at 17:18
• I was going to suggest putting it a little further out than Venus, say in a 300 day orbit -- but this is the essence of it. +1 – Zeiss Ikon Jul 17 at 17:18
• @ZeissIkon You could, but you don't have to. You could get a planet like the OP describes even at Earth's orbital distance if our atmosphere were different enough. – Morris The Cat Jul 17 at 17:21
• Venus possibly didn't start with any Carbon Dioxide at all, I can't think what the article's called off the top of my head but it suggested that Venus was originally a wet world but due to not having its own magnetosphere the young sun ionised its water releasing hydrogen that blew away on the stellar winds and radical oxygen that stripped carbon and sulfur from the crust forming the modern CO2 and sulfuric acid atmosphere. – Ash Jul 17 at 17:22

Models suggest that a desert planet (that is to say, a planet with some polar surface water, but otherwise dominated by land), can remain habitable as close as ~0.75 AU from a star with luminosity of 1 Sol (Abe et al. 2011). This is only a touch further out than Venus's orbit, which has a semi-major axis of 0.723 AU.

However, it is important to consider that main-sequence stars do grow hotter as they age, so if your planet began life near this inner boundary, it may not remain habitable as the star grows older and the habitable zone expands.

To establish where this boundary lies for other classes of star, apply the equation: $$0.75 \sqrt{ L }$$, where L is the star's luminosity.

As was stated already, having less water might allow for your planet to have higher temperatures without triggering a runaway greenhouse effect which would boil away the oceans and cover the world in steam.

And being closer to the sun would make the planet hotter than it would be otherwise.

But there would be more to the climate of a global desert than just having high temperature and low humidity.

Deserts dissipate heat quickly. At night they regularly drop below freezing. Strong temperature differences produce strong pressure differences which in turn produce extreme winds. A global desert without a very dense atmosphere that could transfer heat more efficiently, would have extreme winds between dayside and nightside. Sometimes it would even have global storms.

This would cause much erosion which would produce sand. Now, desert sand has much higher albedo than most of Earth's surface (6-7 times greater that of the ocean) and this is why deserts actually dissipate heat so fast, but the really cool cooling potential of sand on a desert planet is another one: Winds kick up dust and sand which blocks some sunlight temporarily - volcanoes actually do this by ejecting ash and do cool the Earth and it is predicted that meteor impacts would too because they would kick up so much dust. Without rain, dust takes longer to settle down from the atmosphere. So, besides cold nights, there could be cool, dark, dusty days after global storms. Global dust storms are a real thing, at least on Mars - they even hampered the Mars rovers because those were solar-powered. Plants wouldn't be able to get much work done during those days. Just breathing freely could hurt your health because you'd be breathing in all those dust particles.

So, to summarize, not every day could be "hot as all hell", sometimes humans would need air filters (not having them wouldn't kill them quickly, but would hurt their health), and plants would need to be adapted to survive extended periods of low light.

I believe that in theory an Earth-like world can orbit the Sun at any distance beyond about 0.011AU from Earth's orbital track, thus being outside Earth's Hill Sphere when they're at closest approach. In reality you'd need it to be farther in since orbits are rarely perfectly circular and the smaller the gap between the orbital tracks the less stable the orbits will tend to be. How close you need to get the world for the conditions you want is a balancing act between greenhouse gas concentration in the atmosphere, surface water, ice and stellar output.

Any world closer to the sun than Earth will receive more radiation, in proportion to how close it is to its star, but if it's also relatively dry, having little water that cycles through the atmosphere, then other greenhouse gases will be needed to keep it from freezing over anyway. This presents some issues when it comes to having plant life. The first green "plants" (using the term loosely) to evolve on Earth irrevocably altered our atmosphere going away from CO2 as the main greenhouse gas while increasing water vapour and methane instead. Without large amounts of water vapour to fill the gap Earth-like plants wouldn't be a good idea. However before those first green plants evolved there were other photosynthesisers, on Earth they didn't get very far but they could evolve further in an environment with no competition and you have plants that don't effect the atmospheric greenhouse effect.

• "orbits are rarely perfectly circular" I'd be curious to know even one known orbit with eccentricity = 0. Can you enlighten me? I know there are some that are pretty darn close but still elliptical; those don't count. – a CVn Jul 17 at 21:59
• The Hill sphere is not a magical boundary where the gravitational influence of a body stops. A planet orbiting at 0.989 AU would be strongly perturbed by Earth, and it would strongly perturb Earth. The system would be very unstable, with quick, drastic and unpredictable consequences for one or both planets. – AlexP Jul 18 at 7:22
• @aCVn Even Earth has an orbit eccentricity almost zero - at times; it doesn't last long, of course. Or at least so we think. So "temporally rare" would be a fair statement. – Luaan Jul 18 at 7:43
• @AlexP 0.989AU Is further than both Venus and Mercury are from Earth on average, Earth does not perturb either planet's orbit. – Ash Jul 18 at 13:42
• Radius of 0.989 = 1 - 0.011 AU, obviously. (0.011 AU taken from the answer.) – AlexP Jul 18 at 14:50

The problem is run-away greenhouse effect at the inner edge of the "Goldilocks zone". This is defined by the point where insolation causes the planet's cloud cover to approach 100% and its albedo to be close to maximum. Any further increase of solar input causes more water to evaporate but cannot increase cloud cover any more -- and since water vapour is a greenhouse gas, the presence of more of it in the atmosphere raises temperature still more. Positive feedback. The planet suddenly becomes a "cool Venus".

This is the probable fate of life on Earth, as the Sun evolves even hotter. It probably won't happen for over a billion years.

If you try to get around this by starting the planet with very little water, you instead have the problem that you can't have three billion years for life to evolve before all the water vapour is blown out into space by the solar wind and you end up with a "hot Mars" -- a totally dessicated place.

You might be able to get something with plants using some sort of hand-waving (or even by some rather complex modelling). We don't know that three billion years is always necessary to get to multi-cellular plants. Maybe 500 million is enough, elsewhere. And if the star is a bit cooler than our Sun and the planet a bit nearer, the star won't heat up as fast as the Sun has, so you could stay on the inner edge of the Goldilocks zone for longer. I don't know whether a cooler star has a less energetic solar wind that would strip less water, or whether being closer would worse than cancel that and make such a planet lose water faster. (Don't stray into red dwarf territory. The planet would get rotation-locked to a star that close, and red dwarfs are flare stars. Altogether not good for life to evolve).

Finally, it's possible that biological evolution has stumbled into a more powerful negative feedback regulation of the planetary temperature than by water vapour and cloud cover. Perhaps plant life emits a significant amount of a potent greenhouse gas, but this biochemical pathway happens to get progressively inhibited at above 50C. Here on earth, the oceans emit lots more methyl iodide than humans do, and its an enormously powerful greenhouse gas ... but (fortunately) its unstable in an Oxygen atmosphere.

(In passing, I once read that even given a Methane atmosphere back then, Earth should have been too cool to support life 2.5 billion years ago. I wonder whether life back then was also emitting methyl iodide -- it's stable in Methane. )