November 14, 2013
In The Hunt For Habitable Planets, A New Model May Be Key
Lee Rannals for redOrbit.com - Your Universe Online
Researchers using new theoretical models have created a new method to simplify the search for Earth-like planets. The new models help rule out the possibility of Earth-like conditions on certain planets outside of our solar system.
Yann Alibert, of the Center for Space and Habitability (CSH) at the University of Bern, and colleagues based the study on the mass and radius of a planet, helping to determine criteria that exclude the possibility of life as we know it. The data required for the study was provided by the HARPS-spectrograph in Chile, which was developed by the University of Geneva and Bern.
Current observational programs are looking to detect planets outside our solar system that sit in the habitable zone while also featuring the best conditions for life to potentially exist. The question about whether these planets lie in the habitable zone is an easy one to answer, but knowing the conditions of a planet in order for it to be able to host life is another story altogether.
The new model, developed by Alibert, is able to determine whether a planet is inhabitable from the data provided by HARPS and the upcoming space telescope CHEOPS being developed by the European Space Agency and CSH.
“This theoretical model will help astronomers concentrate on promising candidates in their search for Earth-like planets,” said Alibert, lead author of the research published in the journal Astronomy & Astrophysics, said in a statement.
The model is looking for two conditions, without which life as we know it is not possible. These conditions include water in liquid form and a so-called carbon cycle. These conditions help to form the foundation of the theoretical models.
The carbon cycle is a geological process that helps to regulate the CO2 level in the atmosphere and with that, the temperature of the planet’s surface. Carbon dioxide in its dissolved form in the ocean undergoes a chemical reaction and is then transported into the Earth's mantle. High temperatures in the inner parts of the Earth’s mantle help to release CO2 back into the atmosphere during volcanic eruptions.
If a planet’s given mass has a large radius, the density would be very low, meaning there will be no Carbon cycle or liquid water on that planet. Scientists say this is because low density is an indicator for a lot of gas and/or water. If a planet contains a lot of gas, the atmospheric pressure on the surface may be so high that water is not able to keep its liquid form.
When a planet is covered by an immense amount of water, the pressure at the bottom of the ocean will increase so much so that the water occurs in the form of “Ice VII,” which does not exist on Earth. This form of water has such a high density that it settles on the ocean floor, forming a barrier between the rocks on the ocean floor and the water above, preventing the carbon cycle.
“Our study shows that a planet, that consists of a lot of gas or water, is not habitable,” Alibert said.
A planet with the same mass as Earth can have, at maximum, a radius 1.7 times that of Earth’s, including the gas and hydrosphere. A “Super Earth,” or a planet 12 times more massive than Earth, must have a radius 2.2 times Earth’s radius. Alibert says that mainly larger exoplanets have been discovered so far, but in the near future astronomers will be discovering smaller and more promising planets.