We’d love to find another planet like Earth. Not exactly like Earth; that’s kind of ridiculous and probably a little more science fiction than science. But what if we could find one similar enough to Earth to make us wonder?
How could we find it? We progress from one planet-finding mission to the next, compiling a list of planets that may be “Earth-like” or “potentially habitable.” Soon, we’ll have the James Webb Space Telescope and its ability to study exoplanet atmospheres for signs of life and habitability.
But one new study is focusing on exomoons and the role they play in a planet’s habitability. If we find a Moon-like exomoon in a stable orbit around its planet, could it be evidence that the planet itself is more Earth-like? Maybe, but we’re not there yet.
Planetary formation is a complicated, multilayered process. Even with the influx of data on exoplanets, there are still only two known planets that are not yet fully formed. Known as PDS 70b and PDS 70c, the two planets, which were originally found by the Very Large Telescope, are some of the best objects we have to flesh out our planetary formation models. And now, one of them has been confirmed to have a moon-forming disk around it.
The search for life on exoplanets takes a fairly conservative approach. It focuses on life that is similar to that of Earth. Sure, it’s quite possible that life comes in many exotic forms, and scientists have speculated about all the strange forms life might take, but the simple fact is that Earth life is the only form we currently understand. So most research focuses on life forms that, like us, are carbon based with a biology that relies on liquid water. But even with that narrow view, life could still be hiding in places we don’t expect.
Beyond Earth’s only satellite (the Moon), the Solar System is packed full of moons. In fact, Jupiter alone has 79 known natural satellites while Saturn has the most know moons of any astronomical body – a robust 82. For the longest time, astronomers have theorized that moons form from circumplanetary disks around a parent planet and that the moons and planet form alongside each other.
However, scientists have conducted multiple numerical simulations that have shown this theory to be flawed. What’s more, the results of these simulations are inconsistent with what we see throughout the Solar System. Thankfully, a team of Japanese researchers recently conducted a series of simulations that yielded a better model of how disks of gas and dust can form the kinds of moon systems that we see today.
In order to be considered habitable, a planet needs to have liquid water. Cells, the smallest unit of life, need water to carry out their functions. For liquid water to exist, the temperature of the planet needs to be right. But how about the size of the planet?
Without sufficient mass a planet won’t have enough gravity to hold onto its water. A new study tries to understand how size affects the ability of a planet to hold onto its water, and as a result, its habitability.
Astronomers have discovered, for the first time, moons forming in the disk of debris around a large exoplanet. Astronomers have suspected for a long time that this is how larger planets—like Jupiter in our own Solar System—get their moons. It’s all happening around a very young star named PDS 70, about 370 light years away in the constellation Centaurus.
A pair of astronomers combing through data from the Kepler spacecraft have discovered the first exomoon. The moon is in the Kepler 1625 system about 8,000 light years away, in the constellation Cygnus. It orbits the gas giant Kepler 1625b, and, unlike all the moons in our Solar System, this one is a “gas moon.”
It was only a matter of time before we found an exomoon. We’ve found thousands of exoplanets, thanks mostly to the Kepler spacecraft. And where there are planets, we can expect moons. But even though it seemed inevitable, the first confirmed exomoon is still exciting.
Astronomers and planetary scientists thought they knew how to find evidence of life on planets beyond our Solar System. But, a new study indicates that the moons of extrasolar planets may produce “false positives” adding an inconvenient element of uncertainty to the search.
More than 1,800 exoplanets have been confirmed to exist so far, with the count rising rapidly. About 20 of these are deemed potentially habitable. This is because they are only somewhat more massive than Earth, and orbit their parent stars at distances that might allow liquid water to exist.
Astronomers soon hope to be able to determine the composition of the atmospheres of such promising alien worlds. They can do this by analyzing the spectrum of light absorbed by them. For Earth-like worlds circling small stars, this challenging feat can be accomplished using NASA’s James Webb Space Telescope, scheduled for launch in 2018.
They thought they knew how to look for the signature of life. There are certain gases which shouldn’t exist together in an atmosphere that is in chemical equilibrium. Earth’s atmosphere contains lots of oxygen and trace amounts of methane. Oxygen shouldn’t exist in a stable atmosphere. As anyone with rust spots on their car knows, it has a strong tendency to combine chemically with many other substances. Methane shouldn’t exist in the presence of oxygen. When mixed, the two gases quickly react to form carbon dioxide and water. Without some process to replace it, methane would be gone from our air in a decade.
On Earth, both oxygen and methane remain present together because the supply is constantly replenished by living things. Bacteria and plants harvest the energy of sunlight in the process of photosynthesis. As part of this process water molecules are broken into hydrogen and oxygen, releasing free oxygen as a waste product. About half of the methane in Earth’s atmosphere comes from bacteria. The rest is from human activities, including the growing of rice, the burning of biomass, and the flatulence produced by the vast herds of cows and other ruminants maintained by our species.
By itself, finding methane in a planet’s atmosphere isn’t surprising. Many purely chemical processes can make it, and it is abundant in the atmospheres of the gas giant planets Jupiter, Saturn, Uranus, and Neptune, and on Saturn’s large moon Titan. Although oxygen alone is sometimes touted as a possible biomarker; its presence, by itself, isn’t rock solid evidence of life either. There are purely chemical processes that might make it on an alien planet, and we don’t yet know how to rule them out. Finding these two gases together, though, seems as close as one could get to “smoking gun” evidence for the activities of life.
A monkey wrench was thrown into this whole argument by an international team of investigators led by Dr. Hanno Rein of the Department of Environmental and Physical Sciences at the University of Toronto in Canada. Their results were published in the May, 2014 edition of the Proceedings of the National Academy of Sciences USA.
Suppose, they posited, that oxygen is present in the atmosphere of a planet, and methane is present separately in the atmosphere of a moon orbiting the planet. The team used a mathematical model to predict the light spectrum that might be measured by a space telescope near Earth for plausible planet-moon pairs. They found that the resulting spectra closely mimicked that of a single object whose atmosphere contained both gasses.
Unless the planet orbits one of the very nearest stars, they showed it wasn’t possible to distinguish a planet-moon pair from a single object using technology that will be available anytime soon. The team termed their results “inconvenient, but unavoidable…It will be possible to obtain suggestive clues indicative of possible inhabitation, but ruling out alternative explanations of these clues will probably be impossible for the foreseeable future.”
I firmly believe that our next greatest discovery will be detecting an exomoon in orbit around a distant exoplanet. Although no one has been able to confirm an exomoon — yet — the hunt is on.
Now, a research team thinks following a trail of radio wave emissions may lead astronomers to this groundbreaking discovery.
The difficulty comes in trying to spot an exomoon using existing methods. Some astronomers think that hidden deep within the wealth of data collected by NASA’s Kepler mission are miniscule signatures confirming the presence of exomoons.
So the team, led by Ph.D. student Joaquin Noyola, from the University of Texas at Arlington, decided to look a little closer to home. Specifically, Noyola and colleagues analyzed the radio wave emissions that result from the interaction between Jupiter, and it’s closest moon, Io.
During its orbit, Io’s ionosphere interacts with Jupiter’s magnetosphere — a layer of charged plasma that protects the planet from radiation — to create a frictional current that emits radio waves. Finding similar emissions near known exoplanets could be the key to predicting where moons exist.
“This is a new way of looking at these things,” said Noyola’s thesis advisor, Zdzislaw Musielak, in a press release. “We said, ‘What if this mechanism happens outside of our Solar System?’ Then, we did the calculations and they show that actually there are some star systems that if they have moons, it could be discovered in this way.”
The team even pinpointed two exoplanets — Gliese 876b, which is about 15 light-years away, and Epsilon Eridani b, which is about 10.5 light-years away — that would be good targets to begin their search.
With such a promising discovery on the horizon, theoretical astronomers are beginning to address the factors that may deem these alien moons habitable.
“Most of the detected exoplanets are gas giants, many of which are in the habitable zone,” said coauthor Suman Satyal, another Ph.D. student at UT Arlington. “These gas giants cannot support life, but it is believed that the exomoons orbiting these planets could still be habitable.”
Of course one look at Io shows the drastic effects a nearby planet may have on its moon. The strong gravitational pull of Jupiter distorts Io, causing its shape to oscillate, which generates enormous tidal friction. This effect has led to over 400 active volcanoes.
But a moon at a slightly further distance could certainly be habitable. A second look at Europa — Jupiter’s second-most inner satellite — demonstrates this facet. It’s possible that life could very well exist under Europa’s icy crust.
Exomoons may be frequent, habitable abodes for life. But only time will tell.
The findings have been published in the Aug. 10 issues of the Astrophysical Journal and are available online.
Astronomers believe that hidden deep within the wealth of data collected by NASA’s Kepler mission are minuscule signatures confirming the presence of exomoons. With such a promising discovery on the horizon, researchers are beginning to address the factors that may deem these alien moons habitable.
A new study led by Dr. René Heller from McMaster University in Canada and Dr. Jorge Zuluaga from the University of Antioquia in Colombia takes a theoretical look at habitability – exploring the key components that may make exomoons livable. While stellar and planetary heating play a large role, it’s quickly becoming clear that the magnetic environments of exomoons may be even more critical.
An exoplanet’s habitability is first and foremost based on the circumstellar habitable zone – the temperature band around a star in which water may exist in its liquid state. Exomoons, however, have an additional set of constraints that affect their habitability. In a set of recent papers, Dr. Heller and Dr. Rory Barnes (from the University of Washington) defined a “circumplanetary habitable edge,” which is roughly analogous to the circumstellar habitable zone.
Here the question of habitability is based on the relationship between the exomoon and its host planet. The additional energy source from the planet’s reflected starlight, the planet’s thermal emission, and tidal heating in the moon may create a runaway greenhouse effect, rendering the exomoon uninhabitable.
One look at Io – Jupiter’s closest Galilean satellite – shows the drastic effects a nearby planet may have on its moon. The strong gravitational pull of Jupiter distorts Io into an ellipsoid, whose orbit around the giant planet is eccentric due to perturbations from the other Galilean moons. As the orbital distance between Jupiter and Io varies on an eccentric orbit, Io’s ellipsoidal shape oscillates, which generates enormous tidal friction. This effect has led to over 400 active volcanic regions.
Note that this is an edge, not a zone. It defines only an innermost habitable orbit, inside which a moon would become uninhabitable. The exomoon must exist outside this edge in order to avoid intense planetary illumination or tidal heating. Exomoons situated in distant orbits, well outside the circumplanetary habitable edge, have a chance at sustaining life.
But the question of habitability doesn’t end here. Harmful space radiation can cause the atmosphere of a terrestrial world to be stripped off. Planets and moons rely heavily on magnetic fields to act as protective bubbles, preventing harmful space radiation from depleting their atmospheres.
With this in mind, Heller and Zuluaga set out to understand the evolution of magnetic fields of extrasolar giants, which are thought to affect their moons. It’s unlikely that small, Mars-sized exomoons will produce their own magnetic fields. Instead, they may have to rely on an extended magnetic field from their host planets.
This planetary magnetosphere is created by the shock between the stellar wind and the intrinsic magnetic field of the planet. It has the potential to be huge, protecting moons in very distant orbits. Within our own Solar System Jupiter’s magnetosphere ends at distances up to 50 times the size of the planet itself.
Heller and Zuluaga computed the evolution of the extent of a planetary magnetosphere. “Essentially, as the pressure of the stellar wind decreases over time, the planetary magnetic shield expands,” Dr. Heller told Universe Today. “In other words, the planetary magnetosphere widens over time.”
The team applied these two models to three scenarios: Mars-sized moons orbiting Neptune-, Saturn-, and Jupiter-like planets. These three systems were always located in the center of the circumstellar habitable zone of a 0.7 solar-mass star. Here are the take-home messages:
1.) Mars-like exomoons beyond 20 planetary radii around any of the three host planets act like free planets around a star. They are well outside the habitable edge, experiencing no significant tidal heating or illumination. While their extreme distance is promising, they will never be enveloped within their host planet’s magnetosphere and are therefore unlikely to harbor life.
2.) Mars-like exomoons between 5 and 20 planetary radii face a range of possibilities. “Intriguingly, formation theory and observations of moons in the Solar System tell us that this is the range in which we should expect most exomoons to reside,” explains Dr. Heller.
For an exomoon beyond the habitable edge of a Neptune-like planet it may take more than the age of the Earth, that is, 4.6 billion years to become embedded within its host planet’s magnetosphere. For a Saturn-like planet it may take even longer, but for a Jupiter-like planet it will take less than 4.3 billion years.
3.) Mars-like exomoons inside 5 planetary radii are enveloped within the planetary magnetosphere early on but not habitable as they orbit within the planet’s habitable edge.
In order for an exomoon to be habitable it must exist well outside the habitable edge, safe from stellar and planetary illumination as well as tidal heating. But at the same time it must also exist near enough to its host planet to be embedded within the planet’s magnetosphere. The question of habitability depends on a delicate balance.
Dr. Zuluaga stressed that “one of the key consequences of this initial work is that although magnetic fields have been recognized as important factors determining the habitability of terrestrial planets across the Universe, including the Earth, Mars, and Venus, in the case of moons, the magnetic environment could be even more critical at defining the capacity of those worlds to harbor life.”
The paper has been accepted for publication in the Astrophysical Journal Letters and is available for download here.