Tau Ceti and Its Habitable Worlds

Astronomers have known for some time now that Tau Ceti, a star found in the constellation Cetus, is very similar to our own Sun, though it’s not quite as big at only 78 percent the size, and just a bit less bright. And, due to a larger amount of space dust and a substantial “debris disk,” any planet found there would be subject to a lot more bombardment than planets in our own solar system, which would prove hazardous to any creatures who might have evolved there.

The good news is, the five planets that were suspected to exist since December of 2012 have recently been confirmed. These planets are between two and six times the mass of earth, with the fastest one orbiting its star in just 14 days while the slowest takes 640.

The most promising of these in terms of the search for life would be HD 10700e, which orbits at a distance of about 0.5 astronomical units–or half as far as Earth is from our Sun. Because Tau Ceti is not as big, its habitable zone is closer to its star than our own. This planet moves around its star every 168 days and has a mass over 4 times that of Earth. It’s temperature is a bit on the hot side so only life that could thrive in such heat would be possible.

HD 10700f, on the other hand, would be on the cold side since it’s farther away from the star, moving more slowly as it orbits Tau Ceti once every 640 days. Only a strong greenhouse effect would warm the planet enough for life as we know it to potentially develop there.

Even though a planet occupies the correct zone, that doesn’t necessarily mean it possesses the other qualities necessary to harbor life. One potential difficulty in that regard is that Tau Ceti is metal-deficient. This usually means a star would have very few large planets, and those which did form would be less likely to be terrestrial in nature.

Tau Ceti is just under 12 light-years from our own Sun and is considered a high-proper-motion star. It resides inside what is called a “galactic habitable zone.”

A galactic habitable zone–which differs from the circumstellar habitable zone surrounding each star and refers to the portion of the galaxy most likely to harbor life– incorporates a variety of factors, including metallicity, rate of major catastrophes such as supernovae, and the types of stars found there, to calculate which systems are most likely to have formed terrestrial planets. That’s how astronomers figure out which systems to look at for possibilities of life or planets capable of supporting life.

How do scientists measure the potential to develop and sustain life on a planet or a natural satellite? First, they agree that life could develop naturally or it could be transferred from another body, making the search quite difficult. But, since the only place we know for a fact that has life is our own planet, they decided the best way to find more life would be to seek worlds very much like our own, circling a similar star.

More than one theory exists on how we should search. Some scientists theorize that due to a concept called the Rare Earth hypothesis, complex multicellular life developed on Earth because of an improbable combination of astrophysical and geological circumstances, and, therefore, any other complex life would require similar and quite rare circumstances.

Others follow the logic of the principle of mediocrity: Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. These scientists feel the universe has plenty of complex life and we only need to locate it.

Either way, the current search has involved locating terrestrial planets that can retain liquid water, maintain a stable atmosphere, and a temperature that is not so hot it would melt the life form or so cold that it would freeze it.

However, these theories have led to other hypotheses like the one that says a planet need not have an atmosphere if liquid water exists deeper within or perhaps life forms unlike those we understand could exist in places life from our own world could never develop or even survive.

Did you know that yellow giants are not the only stars with potential planets? Red Dwarf stars could have potential too.

[Image Via zmscience.com]