Scientists develop a new way to find potentially habitable planets outside of our solar system

The question of ‘are we alone in the universe’ is usually the preserve of philosophers but now a team of scientists may have a way to find the answer. They hope to create a list planets that could be capable of sustaining ‘complex alien life’ to help astronomers know where to point their telescopes. Thousands of exoplanets…

The question of ‘are we alone in the universe’ is usually the preserve of philosophers but now a team of scientists may have a way to find the answer. 

They hope to create a list planets that could be capable of sustaining ‘complex alien life’ to help astronomers know where to point their telescopes. 

Thousands of exoplanets – planets orbiting a star outside our solar system – have been discovered since the first one was found in 1992.

The search is now moving towards finding which of those planets might be able to sustain life in the hope of answering the question, ‘Are we alone in the universe?’

Researchers focused their studies on planets orbiting red dwarf stars as they are the most common type of star in the universe. They discovered that a star’s radiation plays a ‘deciding factor’ in the habitability of planets orbiting it

The new study by a team at Northwestern University in Illinois involved examining exoplanets orbiting red dwarf stars, to determine the impact the radiation of a star and its planet’s rotation rate have on habitability.

Lead author, Howard Chen said: ‘If we can predict which planets are most likely to host life, then we might get that much closer to answering the question of whether we are alone in the universe within our lifetimes.’ 

They found that a star’s radiation is a ‘deciding factor’ in whether or not a planet is habitable.

Using a combination of 3D climate modelling and chemistry analysis, they discovered how active a star is and the size of a planet’s ozone layer also play a part.

This is because radiation from an active star could lead to surface water being vaporised, while a thin ozone would let in too much radiation, making the planet hazardous for complex surface life. 

This allowed them to narrow the list of life sustaining planets by removing those around very active stars and planets with very thin ozone layers.

Mr Chen said: ‘Instruments, such as the Hubble Space Telescope and James Webb Space Telescope, have the capability to detect water vapour and ozone on exoplanets. They just need to know where to look.’

The study focuses on red dwarf stars, also known as M Dwarfs as these are the most common types of stars found in the Milky Way, making up about 70% of the stars in our galaxy. 

Our sun is a yellow dwarf, which is a type of main sequence star that account for about 7% of the stars found in the Milky Way.

A typical Red Dwarf is 50 times dimmer than the Sun and just 10 to 20 per cent the size of our star. 

For a planet to sustain complex life it has to be able to retain liquid water. 

If it is too close to the star then water will vaporise, if it is too far the water will freeze and if that happens the surface won’t be warm enough to sustain complex life. 

WHAT IS THE JAMES WEBB SPACE TELESCOPE? 

The James Webb telescope has been described as a ‘time machine’ that could help unravel the secrets of our universe.

The telescope will be used to look back to the first galaxies born in the early universe more than 13.5 billion years ago, and observe the sources of stars, exoplanets, and even the moons and planets of our solar system.

The vast telescope, which has already cost more than $7 billion (£5 billion), is considered a successor to the orbiting Hubble Space Telescope

 When it is launched in 2020, it will be the world’s biggest and most powerful telescope, capable of peering back 200 million years after the Big Bang. 

The planets in the middle are the ones most likely to be able to sustain complex life, an area known as the Goldilocks Zone after the fairy tale Goldilocks and the Three Bears where Goldilocks finds one porridge too hot, one too cold and one ‘just right’.

The team have been trying to work out how close is too close for a planet to sustain liquid water. They are trying to find the ‘inner edge’ of the habitable zone.

‘The inner edge of our solar system is between Venus and Earth,’ Mr Chen explained. ‘Venus is not habitable; Earth is.’ 

The team found that many planets in the habitable zone could not sustain life due to their thin ozone layers. 

Despite having otherwise habitable surface temperatures, these planets’ ozone layers allow too much ultraviolet radiation to pass through and penetrate to the ground. The level of radiation would be hazardous for surface life.

Daniel Horton, senior researchers on the project, said: ‘There are a lot of stars and planet out there, which means there are a lot of targets. Our study can help limit the number of places we have to point our telescopes.’ 

Previously scientists have used one dimensional and two dimensional global climate models to study Red Dwarf planets but have not taken the chemistry into account, so have not been able to give as true of a picture, says Mr Chen.

This Earth sized planet called HD 21749c, orbits a star about 53 light-years from Earth but is likely too hot to sustain life. Scientists from Northwestern University hope their research could better direct telescopes to more likely life sustaining candidates in future

‘3D photochemistry plays a huge role because it provides heating or cooling, which can affect the thermodynamics and perhaps the atmospheric composition of a planetary system.’  

The research was conducted with support from researchers at the University of Colorado Boulder, NASAs Virtual Planet Laboratory and the Massachusetts Institute of Technology. 

Mr Chen said for most of human history, the question of whether or not life exists elsewhere has belonged only within the philosophical realm.

‘It’s only in recent years that we have had the modelling tools and observational technology to address this question.’

HOW DO SCIENTISTS STUDY THE ATMOSPHERE OF EXOPLANETS?

Distant stars and their orbiting planets often have conditions unlike anything we see in our atmosphere. 

To understand these new world’s, and what they are made of, scientists need to be able to detect what their atmospheres consist of.  

They often do this by using a telescope similar to Nasa’s Hubble Telescope.

These enormous satellites scan the sky and lock on to exoplanets that Nasa think may be of interest. 

Here, the sensors on board perform different forms of analysis. 

One of the most important and useful is called absorption spectroscopy. 

This form of analysis measures the light that is coming out of a planet’s atmosphere. 

Every gas absorbs a slightly different wavelength of light, and when this happens a black line appears on a complete spectrum. 

These lines correspond to a very specific molecule, which indicates it’s presence on the planet. 

They are often called Fraunhofer lines after the German astronomer and physicist that first discovered them in 1814.

By combining all the different wavelengths of lights, scientists can determine all the chemicals that make up the atmosphere of a planet. 

The key is that what is missing, provides the clues to find out what is present.  

It is vitally important that this is done by space telescopes, as the atmosphere of Earth would then interfere. 

Absorption from chemicals in our atmosphere would skew the sample, which is why it is important to study the light before it has had chance to reach Earth. 

This is often used to look for helium, sodium and even oxygen in alien atmospheres.  

This diagram shows how light passing from a star and through the atmosphere of an exoplanet produces Fraunhofer lines indicating the presence of key compounds such as sodium or helium 

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