# Ask an Astronomer - Extraterrestrial Life

## Are there planets where it's always day/always night?

Published on 28/01/2015
Question:

Have any planets been observed to rotate at the rate that would keep only one  side facing their sun continuously? Do planets rotate at some mathematical constant related to their size and length of orbit around their sun?

This is a great question, planets rotating at the same rate as their star would have one side be permanently day and one side be permanently night as you have said. We have indeed found planets outside out solar system that work in this way. This effect is known as tidal locking and already happens with the moon where one side always faces the earth. (There is a small "wobble" with the moon where it does slightly change as it rotates about its own axis but the same side always faces the earth giving us a "dark side of the moon". You can see a great visualisation here : https://www.youtube.com/watch?v=vC7odtQHoPc ) In the solar system lots of moons are locked to their host planets.

If we imagine that we have a "host" body which might be a star or planet and an orbiting body around it. The main idea with tidal locking is that one half of the body that is orbiting its host is feeling more gravitational pull than the other causing a "bulge" on one side, facing the host. When the orbiting planet (or moon) is very close to its host this bulge will start to lead or lag the bodies orbit, depending on the system. This additional gravitational pull on one side serves to slow down the day-night rotation of the planet or moon until eventually it synchronises with the orbit around the host.

Only some bodies that are close enough are really susceptible to tidal locking. For it to really work you need to be close enough for the bulge to be able to be pulled off-centre the body orbits.  Tidal locking also takes time to set in. When the planets and moons initially form they have their own rotation which over time decays. The time it takes to become locked depends on a few factors including the mass of the objects and the separation of the objects. This means that very close objects that are much lighter than their hosts lock quicker.

All in all tidally locked objects are close to their hosts. We have found some exoplanets that we believe are tidally locked. (For great examples try looking up hot Jupiters like Wasp-43b and HD209458b.)

I hope that answers your question, planets being tidally locked is definitely  possible and we believe we have found very close in planets where, as you've said, it is permanently day on one side and permanently night on the other.

## What kind of alien life to look for?

Published on 10/04/2014
Question:

I've been wondering about this for some years now, scientists have been trying to find alien life in paces similar to Earth, they're trying to find a place with water, decent temp, etc.
It's as if they're trying to look for humans :O, but what really confuses me is that 90% if not all of astronomers believe in evolution, so they'll agree to the fact that we (humans) are the way we are because of our environment and the way we've adapted to our surroundings in the last 100,000 years if there was no water, would that mean that we wouldn't be here right now? Through evolution we would've adapted to our surroundings and not needed water to live, wouldn't it be the something with Aliens? If there life started in a place that had no oxygen and no water, they would've just not need oxygen and water to live, so shouldn't we be looking EVERYWHERE? We are searching for places that would support us, and maybe only us, which is probably why we haven't found something yet

The argument that looking for 'Earth-like' life might blind us to finding potentially dramatically different kinds of life that might even be more common from a universal perspective is one that people have put a considerable amount of thought into.

Essentially the issue boils down this - if lifeforms arise that are completely unlike any on Earth today, then what would they be like and how would we detect their presence?  Without an example of a non-Earth-like lifeform that question is pretty much unanswerable. We can (and people have) come up with rough ideas of how organisms with completely alien biochemistries might operate, but there is no real way of testing how accurate those ideas might be.  What we do know however is what life on Earth is like, and one thing that every living thing on Earth needs is water, which only exists in a usable, liquid, form in a fairly narrow temperature range.  Since Earth-like life is the only thing we know how to look for, that is what we are looking for.  If we discover life somewhere else, such as on Jupiter's moon Europa, then we will have a better idea whether all life shares certain basic requirements or whether there are forms of life that are dramatically different from Earth-like life.

## Habitable planets

Published on 08/11/2013
Question:

Occasionally there is some coverage in the media about the continuing discovery of planets around other star systems in the Milky Way, through the work of the Kepler telescope, and other initiatives. Is there a place on the web I can go to, to check what the latest thinking on all of this work is? I'm thinking of an up to date receod of things like numbers of candidate habitable planets discovered, where they are, stuff like that.

Second question, is there any work being done to try to identify such planets around either Alpha Centauri A or B? Is there somewhere I can go to on the web to see what the current thinking and evidence is from any work on these stars and their surrounding planets, and whether any might be in the 'goldilocks' zone for these stars?

There are several websites that maintain catalogues of exoplanet discoveries, with the two most well known being the Extrasolar Planets Encyclopedia (www.exoplanet.eu) run by Jean Schneider at the Paris Observatory, and the Exoplanet Data explorer (www.exoplanets.org) run by NASA, both of which provide facilities for plotting some of the basic parameters of the planets.  Both of those catalogues also have links to other pages about exoplanets, including things like discussion of potential habitable zone planets (though on that particular point be warned that there is no definitive agreement about the location of the habitable zone so you will likely find significant differences between different sources).

Alpha Centauri, as you can imagine being the nearest star system to our own Sun, has been extensively studied for many years to determine whether it has planets.  Last year a roughly Earth-mass planet was reported around Alpha Cen. B orbiting at 0.04AU (about a tenth of the orbital distance of Mercury), however there is considerable controversy surrounding this discovery and it has yet to be confirmed.  The existence of any planets larger than around Neptune has already been ruled out in the Alpha Centauri system, however finding, or ruling out, small, Earth-size, planets at Earth-like orbital distances is exceptionally difficult.  The Wikipedia article on Alpha Centauri Bb actually does a very good job of summarising the current situation.

## Microscopic life on other planets

Published on 27/08/2013
Question:

I was reading lots on the planets and the atmosphere around them. We have detected creatures in the icy lakes and in very hot areas. I was wondering can there be life on other planets. They may not be aliens but some sort of creatures living on other planets. And if so yes, how could they live on other planets?

The most likely kind of 'aliens' are indeed bacteria and other small micro-organisms similar to the ones that we have found in extreme environments on Earth, such as frozen lakes, hot springs and thermal vents.  There are long term plans to send a mission to Jupiter's moon Europa to search for just this kind of life.  We know that Europa has a liquid ocean beneath it's icy surface, and the geological activity induced by the tides from Jupiter may well result in thermal vents very similar to those found on the ocean floor near mid-ocean ridges on Earth.

Pretty much everywhere we look on Earth we find life that has adapted to living there, and we can see places elsewhere in the solar system that look like they may have environments similar to ones we have found life in on Earth.  Sending missions to these places and finding out whether they also have life will tell us a lot about how common life is in the universe, and how life evolved on Earth.  Missions like that are very complicated, and expensive, however, partly as we need to make sure that they don't accidentally take any Earth life with them.  It would be rather annoying to have sent a mission millions of kilometres only to detect bacteria that have hitch-hiked from Earth!

## Formation of Earth and life

Published on 07/11/2012
Question: