To The Habitability Zone and Beyond!

Uncategorized — By on December 17, 2017 at 11:32 am

Article by Sergio Garza

As of today, we have uncovered THOUSANDS of guides on exoplanets (planets revolving around a star other than the sun) since we confirmed the first exoplanet around a pulsar in 1992. Since then, we have discovered planets around other stars in very diverse environments from scorching hot planets to cold-lonely orphaned planets with no parents stars to orbit around. What many people and scientists have more interest in, however, are special exoplanets which happen to be orbiting their parent (or host) star not too far away to freeze but not too close to boil any surface water on the planet.

Not too far but not too close: This is what is called the Habitability Zone. When you hear about “Earth-like” planets, scientists are usually referring to two properties that define it as such: temperature and size. Is this planet receiving enough warmth for its star to have water on its surface? Is this planet more massive than Mars but not as vast as Neptune? If you answer yes to both of these, congratulations! You have yourself a habitable Earth-like planet.

The basic concept underlying this rule-of-thumb seems simple, but reality is not so straight-forward. Keeping that in mind, let’s go and break the misconception that conditions for life can only happen in the habitable zone using one concept – moons.

Technically, in concept we should be talking about exomoons, but as of today, there has not been a confirmed exomoon discovery. However, that shouldn’t deter us because there is an incredible diversity of moons in our own solar system that break the quick-hand rule of habitability zones. You see, habitability zones refer to have having liquid water on its surface, but this idea says nothing about having liquid water underneath the surface. In fact, we have two incredible examples of worlds with liquid water far from the habitability zone, Europa and Enceladus.

Europa is one of the Galilean moons of Jupiter and only slightly smaller and less massive than our own moon, but Europa contains more ocean water than all of the surface water here on Earth! In 2013, the Hubble Space Telescope discovered jets of saltwater shooting into space from cracks on its icy surface. It is believed that there is a vast liquid ocean underneath the icy crust, heated by the ocean floor which itself is heated by the powerful tides caused between its orbit and Jupiter. These tides mechanically heat the unobtrusive ocean by pulling and relaxing the interior of Europa like playdough.These tides also affect the icy crust, causing it to crack open in some places, releasing water like a geyser into space, creating an ocean of warm seawater in an icy moon far away from the Sun.

Enceladus is in a similar geologic position as Europa, except orbiting farther out Saturn and, ironically, more closely studied. What is most exciting about Enceladus is that the Cassini probe analyzed jets of seawater spurting into space and discovered it to be comprised of organic compounds, specifically methane, carbon dioxide, and nitrogen! This is ground-breaking because organic compounds are the building blocks for life and possible signs of its existence as well. They tell us that habitable environments can exist in very different places, outside our own habitable zone. Adding to the diversity of moons in our solar system, there is Jupiter’s moon Io which has hundreds of very active volcanoes, and Jupiter’s moon Ganymede which is larger than Mercury with possibly an underground ocean.

Moons are like little astrophysical laboratories, entities that are also very creative homes for life outside our familiar Earth. It is also very likely that super-massive Jupiter-like exoplanets could have exomoons larger than Mars, or even the Earth itself! Life has a propensity for dramatically adapting to its surroundings with the most extreme cases of environments and biological specimens rightly deserving the title of ‘extremophiles’. When you hear of newly discovered exoplanets, don’t feel hopeless if these otherworldly objects are not considered “habitable”. Given the massive diversity of moons in our own solar system, it shouldn’t be unlikely that there could be even stranger or perhaps more habitable moons conducive to life out there in our universe.


Bibliography

https://www.nature.com/articles/355145a0

https://solarsystem.nasa.gov/planets/europa/indepth

https://www.scientificamerican.com/article/excitement-builds-for-the-possibility-of-life-on-enceladus/

https://arxiv.org/abs/1408.6164

 

 

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