# Designing an Earth-like alien that can survive the harsh conditions of this desert planet [closed]

The planet or orbits a G type star like the Sun that’s 20 percent brighter than the Sun, it orbits once every 130 days at 0.5 AU with an eccentricity of 0.08, and rotates once every 335 earth-days.

Its atmosphere composition and thickness is similar to Earth but with less water vapor, producing an extremely weak greenhouse effect, that’s about $5^o \ C$.

There are small pockets of surface water that freezes and melts during day/night cycle.

I’m mainly talking about more advanced animal, multicellular life in the title, not just bacteria/unicellular life.

• Welcome to worldbuilding. Which kind of life are you aiming at? Life as we know is water based, so you seem ruling out water based organisms. Also, can you be more specific on what problem you are trying to solve? We cannot do all the job in your place. – L.Dutch - Reinstate Monica Jun 15 '18 at 11:44
• After your edit: water with no water vapor is extremely hard to achieve with Earth conditions. And, by the way, water vapor is not the only greenhouse gas. – L.Dutch - Reinstate Monica Jun 15 '18 at 11:50
• Welcome to Worldbuilding! As L. Dutch pointed out it might be helpful if you provide more information of what you exactly envision (a humanoid, bacteria, dinosaur-like beasts or something else). You gave rather specific descriptions of your planet, but barely any description of what your question is aiming at. – ArtificialSoul Jun 15 '18 at 11:55
• Always remember that even desert and ice planets need to have green growy stuff that supplies the oxygen for the space heroes. Makes me sad sometimes to see planets with no plant life and lots of angry animals in films. – KalleMP Jun 15 '18 at 17:12

Depends how big the planet is and the direction of rotation. Like the other answers say, you will have very hot days. Very cold nights are also a probability. Even with the greenhouse effect, leaving something relatively exposed (radiatively) to space for 65 days is going to make it pretty cold.

That said, there will be a narrow band between the day and night portions of the planet that will likely be quite temperate. Within this band, liquid water will exist, and survival at those temperatures will not require any special adaptation beyond what exists on earth. Because the planet is not tidally locked to its star, the band will move over time.

How far these lifeforms will need to move in one day depends on the size of the planet and its rotation direction. Assuming the planet orbits the star and rotates on its axis in the same direction, the band will move at a speed of

$$v_{band}=\cos\left(\ell\right)\left[\frac{\pi d}{T_{orbit}} - \frac{\pi d}{T_{axis}}\right]$$

where $d$ is the planet's diameter and $\ell$ is the latitude. Assuming the planet orbits the star and rotates on its axis in opposite directions, the band will move at a speed of

$$v_{band}=\cos\left(\ell\right)\left[\frac{\pi d}{T_{orbit}} + \frac{\pi d}{T_{axis}}\right]$$

For a planet the same size as earth (d = 7900 miles or 12300 km), rotation in the same direction would cause the band to move 117 mi (188 km) per earth day at the equator, while rotation in the opposite direction would cause the band to move 256 mi (425 km) per earth day at the equator. Assuming lifeforms need to sleep approximately 1/3 of their time and could be moving the rest, that would require a moving speed of 7.3 mph (11.8 km/h) for the co-rotation case and 16 mph (25.7 km/h) for the counterrotation case.

These speeds are not feasible for a human over a lifetime, but something more adapted to the situation could probably manage. Therefore, these lifeforms would probably have some of the following adaptations:

• Long limbs for running
• Ability to sleep for several short periods throughout the day
• Light frames, allowing sleeping members to be carried easily
• Limbs not needed for locomotion to forage and eat while on the move
• Extreme temperature sensitivity to stay in the most optimal part of the twilit zone

Worth noting is that if the planet were smaller than earth, the required travel distance and thus speed would decrease proportionally with the diameter. For this to be manageable for humans or human like creatures, you would need a planet about half the diameter of earth and rotating the same direction as its orbit (walking speed of ~3.5 mph (5.6 km/h)), or the creatures would need to live at a latitude of 60° or greater.

• Are you sure about your calculation? Apart from rotating around itself, the planet also move around the star, and this rotation is faster than the first one. – L.Dutch - Reinstate Monica Jun 15 '18 at 14:54
• I was wrong before because I had forgotten about it, but it's fixed now – Aliden Jun 15 '18 at 15:19
• sorry for bringing it up now... but you are also assuming they stay on the equator.. closer to the poles they can move slower – L.Dutch - Reinstate Monica Jun 15 '18 at 15:26
• Yes I was. I'll add a note to that effect in my answer. Either that, or I'm assuming a cylindrical planet... – Aliden Jun 15 '18 at 15:28

20 percent brighter than the Sun, it orbits [...] at 0.5 AU [...]

With some back of the envelope calculation this means the planes receives 4.8 times the energy we receive on Earth, thus about $4800 \ W/m^2$.

For reference, Mercury receives 6.8 times the energy we receive on Earth.

On top of this its rotation is pretty slow, making for really hot days.

I am pretty sure that, if life ever forms on this planet, it will quickly develop something similar to Anastatica hierochuntica, or Rose of Jericho:

Rose of Jericho, also called resurrection plant, either of two species of unrelated plants known for their ability to survive dessication. The small gray plant curls its branches and seedpods inward in the dry season, forming a ball that opens only when moistened. It can survive for years in that form, though it is sometimes uprooted and blown by the wind like a tumbleweed. If still rooted when moistened, it spreads into a green plant as much as 30 cm (1 foot) wide and bears minute white flowers.

Though this is a plant, something similar could be viable also for an animal, like sporification for bacteria. When the dry ball carried by the winds happen to land in a pocket of water it can return vital, get some food (if present, food will be scattered around water pockets, too), reproduce (partners, if available, will be available for mating around the water pockets) and go dormant again.

This last part will require the reproduction cycle insemination-birth-development to be really quick, so that there are good chances of survival for the offspring.

Even better, the organism should be able to regenerate its entire body from a fragment, like certain starfishes can do. In this way fragmentation during dry wandering becomes an opportunity for spreading.

Ok, so it's really hot during daytime and really cold during nighttime.

A customized GMO for the purpose would need first of all...FUR! Right-o, a nice coat to insulate the skin from the day's UV bombardment and retain body warmth for the night.

This guy should look like an anthroporphic fennec (desert fox), with large ears and acute vision (including night vision) to locate potential threats or prey in a place mostly covered by sands.

Stronger legs to allow jumping away from sudden sand traps.

The 'Fenex' should also need omnivore teething, and a methabolism designed to process poisonous substances as long as they are nutrient. Since the Fenex is not limited by evolution, you can add any useful traits to make him the master of the sands.

The finest sense of smell is also fundamental, especially to locate any whiff of precious, previous humidity. This world may be very dry, but you must concede some oases born through subterranean springs. Without water at all, even the most efficient of organisms that is not an extremophile bacterial colony will die.

Lastly, on a not-so-cute aspect, during a prolonged lack of water, the Fenex could safely drink his own wastes and efficiently separate any toxic substance from the water so that he won't get poisoned. Yes, very yucky, but in a life-or-death situation, picky is not the word.

• Certain desert reptiles do better than drinking their own urine: they dehydrate it to the point it becomes jelly on emission, so that the wasted water is the bare minimum. – L.Dutch - Reinstate Monica Jun 15 '18 at 12:33