Planets without suns float around orphaned in interstellar space. We’ve always assumed they couldn’t support life. Find out why that view is changing.
Everything on Earth revolves around the sun. Our planet orbits the sun, of course, but the sun is also central to everything life does in our world.
All of the energy on Earth is solar energy. Even fossil fuels were created by the sun, having formed from the remains of dead plants and animals millions of years ago.
Plants rely on the sun for their energy and animals eat plants for theirs. Our seasons result from the shifts in Earth’s angle toward the sun. We use the hours of daylight we have during the day to mark them.
PLANTS GET ENERGY FROM THE SUN AND ANIMALS EAT PLANTS
Daylight arrives when our part of the world is facing the sun and nighttime comes when we rotate away from the sun. Our circadian rhythms align with the timing of the sunrise, noon and sunset where we live. When we don’t get enough sunshine, we don’t feel well.
The sun warms our oceans and determines their currents. It also warms our atmosphere, causing the weather we experience.
It’s no wonder humans have traditionally worshipped the sun. We all intuitively sense our close connection and dependence on the star we all orbit together.
ALWAYS IMAGINED LIFE ON PLANETS ORBITING OWN SUN
Naturally, we’ve always imagined that we would discover life on other planets orbiting their own sun. Since the sun is so central to our lives, surely life can only be found in star systems like our own.
That’s always been the assumption and the basis for our search for life outside our solar system. Our search for exoplanets is based almost entirely on observing other stars in the galaxy.
We know that there are planets without suns in our galaxy. For example, astronomers detected the free-floating planet PSO J318.5-22 in interstellar space. They used the Pan-STARRS 1 wide-field survey telescope on Mount Haleakala, on the island of Maui in 2013. (Science is a tough life sometimes!)
OPPHAN GAS PLANET SIX TIMES MORE MASSIVE THAN JUPITER
It’s a gas giant planet about six times more massive than Jupiter and about 80 light-years from Earth. Instead of orbiting a star, it revolves around the centre of the galaxy itself.
Not everyone defines planets the same way. Some scientists define planets as having to be in a planetary system.
They prefer to call bodies like PSO J318.5-22 “planetary-mass objects” or “planemos.” Other names for them include “orphan planets” or “rogue planets.”
MAY HAVE BEEN A BIAS BASED ON OUR DEPENDENCE ON THE SUN
However, we name them, the consensus has always been that they couldn’t possibly support life. This may have been a bias based on our own experience living lives that are wholly dependent on our sun.
A new study in Astrophysical Journal Letters proposes that earth-sized planets without suns could sustain surface water and other liquids. This could happen due to radioactive decay.
As we know, there are radioactive isotopes, or radionuclides, in the Earth’s crust. These include uranium-238, thorium-232, and potassium-40, among others.
RADIONUCLIDES GENERATE POWER IN THE FORM OF HEAT
These isotopes generate power in the form of heat as they decay. Mind you, the sun produces 30,000 times more power, but the energy is there.
A planet that had enough radionuclides and that was close to the centre of the Milky Way Galaxy might have enough heat to prevent freezing on its surface, according to the study. If true, this could change our approach to the search for life on other planets.
Harvard astrophysicist and study co-author Avi Loeb told the journal Science, “That gives you the freedom to be anywhere. You don’t need to be close to a star.”
“YOU DON’T NEED TO BE CLOSE TO A STAR”
The researchers thought about what heat sources planets without suns could rely on instead of stars. The came up with three, the heat left from when they formed, radioactive isotopes that decay quickly and radioactive isotopes that decay slowly.
Then they build computer models to simulate the energy needed to keep water, ammonia and ethane in liquid form on the planet surface. Life would need some sort of liquid solvent to form, and these are the three primary solvents we find in our solar system.
They found that an earth-like planet with about 1,000 times more of both kinds of radioactive isotopes could maintain liquid water over hundreds of millions of years. For liquid ethane, it would only need 100 times more of these isotopes than we have on Earth.
RADIATION LEVELS HUNDREDS OF TIMES HIGHER THAN CHERNOBYL
These planets without suns would still be nasty places to live. The radiation levels would be hundreds of times higher than those seen in the 1986 Chernobyl nuclear disaster.
You probably wouldn’t find any multicellular life such as plants or animals there. If the cells were anything like the ones we know about here on Earth, most of them couldn’t survive there.
Even so, there are some unusually tough, resilient microbes on Earth that might be able to cope with the radiation there. There’s a bacterium called Deinococcus radiodurans that can put up with extreme radiation levels.
UNUSUALLY TOUGH, RESILIENT MICROBES MIGHT BE ABLE TO COPE
Co-author Manasvi Lingam, an astrobiologist at the Florida Institute of Technology, put it this way, “Deinococcus radiodurans is a really crazy organism.” Assuming resistant microbes could evolve, where would these radioactive planets without suns come from.
They wouldn’t be able to form in our galactic neighbourhood. However, since the heavy elements like uranium and others seem to form from neutron star collisions, it could happen near a group of these collapsed stars.
The centre of the Milky Way Galaxy is much more crowded, so there would be more neutron stars there, and they would be more likely to collide. Even if planets without suns with enough radionuclides exist, they would be harder to find than a needle in a haystack.
THEY WOULD BE HARDER TO FIND THAN A NEEDLE IN A HAYSTACK
For one thing, it would be hard to tell these planets from the failed stars that astronomers called brown dwarfs.
One possibility would be to use the new James Webb Space Telescope, which launches in 2021. It could detect a target planet based on its high radiation levels. Still, it would take about ten days, which is a lot of telescope time to divert from other, more promising projects.
If this sounds pretty hypothetical, it is. It’s easier and more efficient to keep concentrating on stars with solar systems in our quest for life.
AN INTRIGUING POSSIBILITY IN THE PRELIMINARY STAGES
Despite this, it’s an intriguing possibility, even though it’s in the preliminary stages. Whether it pans out or not, learning more about this hypothesis would help us to fill in some blanks in the story of how our own planet formed, and our relationships with it and with nature which are both a bit of a mystery.
As Professor Lingam explained, “There are so many unknowns. We haven’t said the last word.”
We always have more to learn if we dare to know.
Science Magazine (AAAS)
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