what makes a planet habitable?

March 8, 2009

Lately, it seems like almost every popular science show about astronomy absolutely has to talk about the habitable zone concept and marvel at how lucky we are to be in just the right orbit around our parent star. Supposedly, the distance from the Sun is just enough to keep our water liquid and give life a chance to appear and diversify on our world. But this concept is actually very misleading. It’s not our orbit that keeps the water on Earth in liquid form. It’s actually the pressure and the greenhouse gases of our atmosphere.

icy planet

According to the math, our planet should be a snowball flirting with 0°C or 32°F temperatures all year round. While that’s warm enough for a lot of microbes to exist, complex life probably wouldn’t have evolved. Add to that the fact that the Sun was actually much colder a billion years ago than it is today and it seems that without greenhouse gases, Earth would be a frozen desert. Clearly, there’s a major problem with using the distance from the star to determine the habitability of a planet. With the recent launch of Kepler to look for other Earths in other solar systems, we need to keep that in mind and develop a much more precise approach to figure our whether life like we know and understand today could evolve on one of the planets we’re bound to detect, and give climate its due as a key player in the evolutionary process.

This is exactly what an interdisciplinary team of scientists said in a 2008 paper on the subject of life on alien worlds. Without directly rebuking the habitable zone concept, they pointed out a wide number of flaws which have to be taken into consideration when a new planet which looks like a potential habitat for extraterrestrials is found. For example, if the planet has a very thick atmosphere and high pressures, water remains liquid at temperatures above 100°C since the boiling point of water on Earth is based on the pressure of our atmosphere at surface level. We can see examples of this phenomena in ocean trenches where volcanic eruptions can’t boil the water around them due to the immense pressure on the bottom of an ocean.

The team also outlines biological considerations. We know living things that feel comfortable at temperatures we would find to be inhospitable. Bacteria can live in -85°C while some strains of archaea could withstand 130°C and reproduce at -10°C and 121°C respectively. Sure, it’s not intelligent, complex life that many space enthusiasts and sci-fi fans want to find, but it’s life by all existing definitions. Because we don’t really know what alien life is like, we can’t rule out that it may be even tougher that that. After all, natural selection on an alien planet would do its part in weeding out organisms that aren’t efficient enough to thrive in harsh conditions.

But the most important aspect of their work is pointing out that seasons and regional climates would play a major role in influencing the planet’s habitability. How well does the planet hold up to an ice age? In an ice age, is everything frozen or are there places where temperatures would be nice and warm for complex life? Understanding the dynamics of climates on an alien planet, the long term trends of its orbit and the duration of its seasons are all major factors in making a case for a specific kind of life on a recently detected planet. Casual eyeballing of where it is in relation to its parent star just won’t do for planets other than our stellar twins.

See: David Spiegel, Kristen Menou, Caleb Scharf (2008). Habitable Climates The Astrophysical Journal, 681 (2), 1609-1623 DOI: 10.1086/588089

Share on FacebookTweet about this on TwitterShare on RedditShare on LinkedInShare on Google+Share on StumbleUpon
  • MarkC

    “Its not our orbit that keeps the water on Earth in liquid form. Its actually the pressure and the greenhouse gases of our atmosphere.”

    And let’s not forget the magnetosphere, eh? Look what happened to Mars with its dead core.

  • Recommend taking a look at “Rare Earth” and “The Life and Death of Planet Earth” by Peter Ward and Don Brownlee. There are a great many factors…

  • kristi

    so can somebody just briefly explain to me the makes earth an habitable planet

    ( cause i kinda can’t understand it )

  • Anonymous


  • Doberman211

    Okay i see many of you are confused. None of you are wrong, you are all right. But the truth is, you are missing the point. If you think of the Earth as a fluke or pure chance than it seems easier. Here’s in summary what makes our little blue marble habitable:

    1. The orbit needs to be within the habitable zone of the Sun, an area just after Venus and stretching out a little beyond the orbit of Mars. Within this habitable zone, it is not too hot that greenhouse effects (like on Venus) will occur, and not too cold that it would freeze the oceans. Others sometimes call it the Goldilocks zone. All the elements like carbon and oxygen are found here and are essential for life. And there are other things to like if a gas giant moon was orbiting a planet within this zone it would remove the size factor by half. (ex: place Jupiter system between Venus and Earth and you would have 3 new habitable planets. Io would probably become more of a Venus.)

    2. The size of Earth also is important. the reason for Mars being turned into a desert world is because of just that. It was too small to generate sufficient internal heating to power its magnetosphere. We are lucky because in early Earth, we were hit with a planetoid the size of Mars. This was also what created the Moon. The cores of the two planetoids combined creating our solid inner core and liquid outer core. That made it large enough to create a magnetic field powerful enough to stop the deadly cosmic rays that would have sterilized any life trying to evolve. The planet also needs to have enough gravity to hold on to an atmosphere and create the pressure we feel today. though life would still exist if there were slight changes in gravity it just wouldn’t look like any life we would see today.

    3. Luck is important too. if not for it, we would probably have never evolved because of some serious NEOs that would have slammed into the planet and eradicated all life here. Like the dinosaurs for example. but we have found larger objects with Earth-crossing orbits that could wipe out everything and leave the planet a hot desert and then an ice age, and well every natural disaster you can think of about 10 times worse.

    The major important factors are the orbit and the size. remember that. the orbit will see than all the elements for life are there and that it is the perfect temperature. The size is important because of gravity. Luck is just a philosophical thing because i truly believe we are one of the fluke planets of the universe created by accident. I believe in aliens, but not so much as the Star Wars universe. there may be about a couple hundred intelligent species in the Milky Way.

    Just what i can think of off the bat. there are more factors, but these are the important ones that make a planet habitable. The Earth habitable is so much more, but size and orbit are the most important for any form of life and habitability. You can’t have a habitable Pluto no matter where you place it in the Solar System, and you cant have a habitable Earth if you place it in the orbit of Pluto. that is my common sense on the subject.

    Peace out.

  • In my religion, Islam, we are taught that God is the One Who made the so subservient and habitable for us. It is one of His many bounties upon us. Today when we see how there are so many other planets but how none of them even come close to the earth then we should realize how great a bounty this is. And consequently how grateful we have to be.

  • WORD! Universe and favorable conditions for live are SOOO COMPLEX…Don’t be surprised to find living things on one of Jupiter’s moon[Titan] that rains with methane liquid :) You never know!!