by Elizabeth Fernandez
Just over 20 years ago, the planetary dance around us was the only one we knew. By looking at our solar system, we ascertained that small terrestrial worlds formed close to their star, and farther away, large, gaseous giants orbited. Farther still stood the ice worlds, planets enshrouded in cold, frigid clouds. This was the way of the planets. This is what we knew.
All of this changed in 1992, when two planets were found orbiting a pulsar, and again in 1995, when, for the first time, we found a planet orbiting another star. The star was an ordinary star, not too dissimilar to our sun, called 51 Pegasi. The discovery of this planet turned everything we understood about planet formation on its head. Here was a massive planet, at least half the size of Jupiter, orbiting so close to its parent star that its year was only just over four days long. Clearly, we had to rethink what we assumed about planet formation.
But these planets were only the beginning. In the coming years, hundreds of other planets around stars across the galaxy were discovered. Many of these systems involved a massive planet, sometimes much more massive than Jupiter, orbiting closer to its star than Mercury in our own solar system… so close to its parent star that its year was only days long, causing the planet to be a sizzling inferno. Most of these discoveries were made using a technique called Doppler spectroscopy. This technique relies on a planet being massive enough and close enough to its parent star that every time the planet orbits, it exerts a small gravitational tug on its star, causing the star to wobble. The larger the planet is and the closer it is to its star, the bigger the wobble, and the easier it is to detect. So it’s no surprise that many of the planets detected were large and close to their parent stars.
So a question began to form in every astronomer’s mind – was the fact that we were finding buckets of large “hot Jupiters” and very few planets smaller than Neptune a selection effect caused by the detection method, or were solar systems like our own, with smaller rocky worlds closer in and gas giants farther out, exceedingly rare?
In order to answer this question, different, more sensitive detection methods were needed.
Enter the space telescope Kepler.
Kepler was launched in 2009 with one purpose – to find worlds orbiting other suns. And not merely the gas giants. The scientists and engineers behind Kepler wanted to be able to find a terrestrial planet within its star’s habitable zone, a planet close enough to be warmed by its stars rays but not scorched, a planet that one could hypothetically walk on. The search for Earth 2.0 was on.
Instead of looking for the wobble of a star caused by a planet, the scientists behind Kepler hoped to find planets using the transit method. When an extrasolar planet crosses in front of its host star, the star is slightly dimmed as the planet blocks a small amount of light. The larger the planet, the more light is blocked. Hence, if you have an extremely powerful telescope and happen to be looking at a star right when an extrasolar planet crosses in front of it, and if your telescope is sensitive enough, you can just begin to hope that you can see a slight dip in the amount of light you receive. Kepler is sensitive enough to see when the light of the parent star is dimmed by even a minuscule fraction of a percent. And it was vastly successful. Kepler found 4,696 potential planets orbiting other stars, and confirmed 1,030 of these.
Suddenly, our understanding of planetary systems exploded. The known planets were not merely hot Jupiters, but now there were hundreds of objects the mass of Neptune, and even smaller – about the mass of Earth.
These newly discovered planets come in all sorts of sizes and orbital characteristics. Some are puffy gas giants with years that last only a few days. One is a dense world, tidally locked to its parent star (where one side is always facing the star). Temperatures on the surface are hot enough to melt lead, and therefore it is covered with massive oceans of molten rock. Another planet is likely made of diamond, and some have two suns in their skies. Some could potentially be “water worlds”, covered with a global ocean. Kepler has even discovered a planet disintegrating as it orbits its parent star, and a dense multiple planet system with five or six planets crammed into orbits smaller than that of Mercury. There are planets with a radius about twice that of our Earth. We think that once the radius of the planet is about 1.5 times the radius of Earth, the composition of the atmosphere transitions to one of mostly hydrogen and helium. There is no solar system analog to these worlds. What are these super-Earths like? Nobody knows.
These worlds are fascinating, but what really gets people excited is finding a planet about the size and density of Earth, smack dab in its star’s habitable zone. Such a planet could be a true Earth 2.0.
“Searching for Earth 2.0 reminds me of my younger days and trying to find a soul mate,” says Natalie Batalha dreamily, one of the co-investigators of the Kepler mission. Such a find would definitely be special – so special that many scientists refer to the habitable zone as the “Goldilocks Zone” – not too hot, not too cold, but just right.
If a planet is within a certain distance from its parent star so that the temperatures are mild enough that water can exist in liquid form, the planet is said to be within the star’s habitable zone. Any farther and all water would be converted to ice. Any closer, and, like Venus, the planet can have a runaway greenhouse effect, where the pressure and temperature are exceedingly high and electrical storms run rampant, so that if you were to step on the surface, you would simultaneously be melted, squished, and electrocuted.
Not the best place to live.
Kepler told us something amazing – about 20% of stars harbor an Earth-sized planet within its star’s habitable zone. If we imagine the galaxy being scaled to the size of the United States, we would merely have to go a quarter of a mile to find the nearest planet within the habitable zone.
The galaxy is literally littered with Earth-sized planets potentially warm enough for liquid water to exist.
Now, the habitable zone says nothing about whether or not liquid water actually does exist on the surface of the planet. It also says nothing regarding if life does or could exist there. In fact, missions like Kepler, which rely on the transiting method to find extrasolar planets, can only give us two variables – the size of the planet and the radius of its orbit. But follow-up observations using the Doppler method (or the amount that the star wobbles from the planet’s gravitational tug) can provide the mass when combined with the transit observations. The mass and the radius of the planet can give the density – and voila! This number gives us a rough idea of the composition of the planet.
How could we find an Earth 2.0? Kepler is a start. But if the planet has life? This requires more detailed observations – something we hope to obtain by direct observations and spectroscopy.
This is where WFIRST comes in.
One of the main missions of WFIRST, which will hopefully launch in the 2020s, is to study these strange worlds. WFIRST will use a coronagraph, or optical elements that will block the harsh glare from the planet’s parent star. By doing this, we just may see a pale dot that remains – the planet itself.
Seeing the planet directly will open up a whole new realm of our understanding of the diversity of the planetary zoo. We won’t see swirls of clouds or coastlines of oceans, but we hopefully will be able to see the planet’s spectra – the distribution of light from the planet by wavelength. And since certain molecules or chemicals emit strongly at a certain wavelengths, we would be able to detect the presence of all sorts of substances – such as water, methane, and metals. We would then be able to understand a bit more about these worlds.
And perhaps, we could even see evidence for life.
Life is inexorably linked with its planet. It is connected with the ecosystem such that life is constantly changing and affecting the planet’s atmosphere and the planet itself. We can see this when we look at the spectra of the Earth and compare it with the spectra of another lifeless planet, for example Venus. The light distribution from these two planets looks markedly different.
When searching for life, there are many chemicals that astronomers can search for within the planet’s atmosphere. Three of these in particular – O2, O3, and N20 – rarely exist apart from life as they would be quickly absorbed. Still, the presence of these molecules could be misleading, and could potentially still exist even on a lifeless planet.
Because of this, multiple lines of evidence would be needed to say with a greater certainty that life indeed exists on a given planet. In order to understand the global signatures of life on other planets, Niki Parenteau, a research scientist at NASA Ames and SETI, takes a look at primitive life on this planet, namely, large microbial mats in Yellowstone National Park. These very primitive microbes give us an opportunity to see how life looked like billions of years ago, and even to understand its origins. And, if life on other planets had similar beginnings, studying these autotrophs may be the key to finding them. These primitive life-forms would give a distinct spectral signature, or color, to the planet when viewed from space.
For example, if a planet were covered in vegetation, we would expect to see a feature called the “red edge”, the result of chlorophyll strongly absorbing visible light but reflecting light in the near-infrared. If the planet was covered in something similar to “purple bacteria”, an early life-form on this Earth, the spectra would show a feature similar to the red edge, but shifted to longer wavelengths. Life-forms such as algae, lichens, and bacteria each have a distinct color signature. In short, life-forms that could cover the surface of an extrasolar planet would change its color, and this color would give us an idea of where this life obtains energy and how it converts energy to biomass.
Scientists often assume that water is needed for life to be present. As a polar molecule, water can act as a solvent – bringing together atoms and molecules in a way that complex molecules necessary for life can form. However, life could be quite different than life on Earth, and could make use of other potential solvents as well. Dr. Morgan Cable, a scientist at JPL, discussed many other kinds of oceans that could form life. Ammonia can also participate in hydrogen bonding, but ammonia is rarely seen without water. Liquid carbon dioxide is also a potential solvent, and can dissolve sugars and PNA, a peptide that acts like DNA. Some argue that petroleum can even be used as a solvent, although analysis on petroleum is difficult to interpret because of its inherent complexities. And even though liquids such as ethane and methane lack solubility, it is possible that organic molecules could be assembled on a shoreline of an alien lake, where molecules move around as the waves go in and out.
But since we don’t understand how life began on our own planet, we have to accept the possibility that life may look drastically different than what we are used to. And what’s worse, we would have no idea how to understand or even find life that looks nothing like life on this planet – with potentially different chemistries, different methods for energy transfer, and different physiologies. Right now, when it comes to understanding life, we only have one data point.
On this Earth, life is ubiquitous. It is found nearly everywhere – from the arctic, to the jungle, to the deepest ocean trenches where it draws energy from geothermal vents, to high in the atmosphere, to lakes of boiling acid. Our search for life on other worlds would tell us if life is also ubiquitous among the stars – and if so, we should prepare to find some very strange life indeed. But even if we don’t find life in every corner of the Universe, we will no doubt learn something about the origins of life on this planet.
And that is very special indeed.
Article © Elizabeth Fernandez, 2015