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Searching for exoplanet moons: astronomers may soon find moons around extra-solar planets, and amateurs could beat the professionals to the punch.


Does life exist elsewhere in this vast cosmos, or is humanity a coincidence of astronomical proportions?

This question has captivated the minds of great thinkers throughout history. What an astounding honor it is to live at a time when the question may finally be answered, for today astronomers are hunting for habitable worlds outside our solar system.

With only one known example of life, astronomers have typically conceived of an Earth-like planet as the most likely place for biology. We're not saying life cannot exist in other environments, just that an ideal starting point in our search would be a planet similar to Earth. But what about a moon?

Science-fiction writers have long toyed with the idea of habitable satellites, perhaps most famously with the forest moon Endor in Stars Wars: Return of the Jedi. Astronomers expect moons to be solid bodies rather than gas giants, and this is favorable for complex life. With exoplanet discoveries booming, astronomers are now embarking on the audacious challenge of searching for exomoons. Scientists often compare Saturn's moon Titan to the primordial Earth. Earth orbits the Sun in the temperate region where liquid water can exist on the surface. But Titan orbits Saturn, which is far outside this habitable zone. Imagine a different scenario where Saturn lies in the habitable zone and Earth orbits in the cold depths of space. In this alternate reality, it's conceivable that life could have evolved on Titan.

Astronomers have become increasingly interested in exomoons. Out of the 340-plus exoplanets now known, 29 lie in the habitable zone of their stars. Why aren't scientists jumping up and down with excitement about this astonishing fact? Well, even if Saturn sat within the Sun's habitable zone, it's difficult to imagine how complex organisms such as trees, birds, and people could evolve in such an environment. But the moons of these planets are a different story.

The Hunt Is On

Nobody has yet discovered an exomoon because it's an extraordinarily difficult task. Most known exoplanets have been found using the radial-velocity technique. This method relies on looking at the parent star's reflex motion due to an orbiting planet's gravity. Despite this technique's success, it's completely insensitive to exomoons, since the planet and any satellites would simply appear as a single mass orbiting a star.

The best method proposed so far is to use the transit method. If an exoplanet has just the right orbital inclination, it crosses the face of its parent star once every orbit. Several dozen exoplanets, most of which are gas giants, have been discovered with this method.

The problem with using transits is that the moon is likely to be very small, and so far the smallest detected transiting exoplanet is the 1.7-Earth-radii CoRoT-Exo-7b (May issue, page 30). Finding even smaller worlds is very tricky. Not only this, but moons can hide in front or behind their host planet, meaning you need a lucky detection.

Fortunately, things may not be so bad. In 1999 Paola Sartoretti (Observatoire de Paris, Meudon, France) and Jean Schneider (Institut d'Astrophysique de Paris, France) predicted that exomoons could be detected indirectly through a timing technique. The satellite doesn't simply orbit the planet; both the planet and moon orbit a common center of mass, meaning the planet wobbles during its orbit around the star. Sometimes the planet's transit occurs slightly earlier than expected and sometimes slightly later. This effect is known as transit time variation, or TTV.

The TTV of an Earth-mass exomoon around a Neptune-mass exoplanet can range from 20 seconds to several minutes, depending on the orbital arrangement. Given that current telescopes are measuring these transit times to a precision of 10 seconds, then one could naively predict that we're ready to find a moon!

But the story goes sour again.

In 1996 Anthony Dobrovolskis and Bill Borucki (both at NASA/Ames Research Center) came up with the idea that a perturbing planet could also cause TTV. It has also become clear that general relativistic effects, non-periodic orbital variations induced by another planet or star, parallax effects, and other influences can produce TTV. Distinguishing between the plethora of possibilities makes it too difficult to claim that one signal is definitely a moon or definitely another planet. No one has yet used TTV to detect an exomoon because too many things can cause this effect.

Right now the best we can do is say what is not there. In 2001 Tim Brown (High Altitude Observatory) and his team, using the Hubble Space Telescope, were able to use transit timing to rule out a 3-Earth-mass moon around HD 209458b. More recently, Rodrigo Diaz (Instituto de Astronomia y Fisica del Espacio, Argentina) and his colleagues made a positive detection of TTV for OGLE-TR111b, but the signal is far too large for a satellite. It seemed as if an exomoon detection was as far off as ever.

Wobbling Exoplanets

This is when I became interested. The challenge of detecting an exomoon is irresistible and I was convinced that the timing method had not been exhausted. While walking through the streets of London on a rainy summer evening in 2008, the moment of inspiration hit me. How long walks induce physicists to conceive so many ideas remains one of the most perplexing problems in modern science.

After frantically noting the idea in my cell phone, the next morning I wrote it out mathematically and was astonished to find that it might actually work. TTV is caused by the planet wobbling during its orbit and thus transiting slightly ahead or behind predictions. But it's not just the planet's position that changes--its velocity does too. The planet has two velocity components, one due to its motion around the star and one due its motion around the planet-moon center of gravity. Although the latter effect is smaller, the perpendicular component of this velocity will vary for each transit.

Depending on where the moon is, sometimes the planet will speed up during the transit and sometimes it will be held back, moving across the star slower than usual. The inevitable consequence is that the transit's duration will also change. This incredibly simple idea gives us the effect known as transit duration variation, or TDV.

TDV offers a completely new way of looking for exomoons. The really neat thing is that it complements TTV so elegantly. TTV and TDV are both due to planetary wobble, but TTV is a spatial effect whereas TDV is a velocity effect, and the two are always out of phase with each other. When we get a maximum TTV effect, we get a zero TDV effect, and vice versa. The two amplitudes tend to take similar values for many common orbital arrangements, and can range between 1 second and several minutes. A perturbing distant planet would typically generate such a tiny TDV effect that the signal from a large moon completely dominates.

TTV and TDV give rise to a unique and distinguishable signature, so we can say, "Yes, that's definitely a moon!" In this young field, however, there still remains a lot of theoretical work to establish if the method can be reliably extended to detect multiple large satellites, such as the Galilean system around Jupiter.

Remarkably, there are hundreds of amateur astronomers who are capable of detecting transiting exoplanets, and any of them could bag the first habitable exomoon. Imagine that we find a transiting Neptune-size planet orbiting a red dwarf on a 35-day period. Its distance would put it smack dab in the middle of the star's habitable zone. The transit of this star would be about 1% (or 10 millimagnitudes) in depth, which is detectable with even a small telescope equipped with a CCD camera. An Earth-mass exomoon around this planet would induce a TTV of 2 minutes and a TDV of 1 minute.

In 2008 Jeffrey Coughlin and his team at New Mexico State University detected a possible long-term timing variation in the transits of Gliese 43Gb, which could be due to a perturbing planet. A large portion of the data came from amateur astronomers.

Many amateurs have already observed transits of 10 mmag to an accuracy of 1 minute, as verified by the catalog on Bruce Gary's Amateur Exoplanet Archive website ( These capabilities should be good enough to detect Earth-mass exomoons, given the right target. With NASA's Kepler space telescope now flying, astronomers are going to find a lot more transiting planets in the next several years (January issue, page 28), which means we could find exomoons as small as Titan and Europa in the next decade.

The quest for exomoons has begun. With the list of known exoplanets now extending beyond hot Jupiters, there is every reason to expect moons to be found soon. The discovery of a habitable rocky moon would certainly be a major landmark in our quest for answering that profound question: Does life exist elsewhere in the universe? We intend to answer that question in our lifetime, and it's a task in which everyone can participate.


In 2001 Tim Brown and his team used the Hubble Space Telescope to exclude Saturn-like rings around HD 209458b. If a transiting exoplanet has rings, it should induce a small dip in the starlight both before and after the main transit event.

Detecting Exomoons

Below: The gravity of a large moon will cause its host planet wobble around the center of mass of the planet moon system. This wobble causes both the timing and duration vary from one orbit to the next, which make astronomers to detect the moon. All are greatly exaggerated for clarity.


Right: When viewed from above, the means its orbital path around the star Is when it passes in front of its host star as views " (shaded transit zone), its position with respect to the center of mass can vary from one orbit to the next.


Transit Timing Variation (TTV)

A: If the planet lies ahead of the center of mass when this center of mass enters the transit zone, the transit will already be underway; the planet entered a few minutes earlier. This means the transit started a few minutes earlier than predicted. B: If the planet is trailing when the center or mass enters the transit zone, it will take the planet several more minutes to enter the zone, so the transit begins later than predicted.


Transit Timing Variation (TDV)

A: If the planet's motion around the center of mass is opposite the overall motion around the host star, the planet will appear to be moving slower in the transit zone, and thus the duration of the transit will be longer than normal. B: If the planet's motion around the center of mass is in the same direction as the overall motion around the host star, the planet will appear to be moving faster, and thus the transit will be shorter.


David Kipping is a theoretical astronomer working toward his Ph.D. at the University College London, England. His research interests include the detection and characterization of extrasolar planets.
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Title Annotation:More Potential Sites for Life
Author:Kipping, David
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2009
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