Astronomers Now Looking For Exomoons Around Exoplanets

by Ian O'Neill on December 14, 2008

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An artist impression of an exomoon orbiting an exoplanet, could the exoplanet's wobble help astronomers? (Andy McLatchie)

An artist impression of a habitable exomoon orbiting an exoplanet, could the exoplanet's wobble help astronomers? (Andy McLatchie)

It looks like astronomers have already grown tired of taking direct observations of exoplanets, been there, done that. So they are now pushing for the next great discovery: the detection of exomoons orbiting exoplanets. In a new study, a British astronomer wants to use a technique more commonly associated with the indirect observation of exoplanets. This technique watches a candidate star to see if it wobbles. The wobble is caused by the gravitational pull of the orbiting exoplanet, revealing its presence.

Now, according to David Kipping, the presence of exomoons can also be detected via the “wobble method”. Track an exoplanet during its orbit around a star to see its own wobble due to the gravitational interaction between the exoplanet/exomoon system. As if we needed any more convincing that this is not already an ‘all kinds of awesome’ project, Kipping has another motivation behind watching exoplanets wobble. He wants to find Earth-like exomoons with the potential for extraterrestrial life…

If you sat me in a room and asked me for ten years over and over again: “If you were an astronomer, and you had infinite funds, what would you want to discover?“, I don’t think I would ever arrive at the answer: the natural satellites orbiting exoplanets.” However, now I have read an article about it and studied the abstracts of a few papers, it doesn’t seem like such a strange proposition.

David Kipping, an astronomer working at the University College London (UCL), has acquired funding to investigate his method of measuring the wobble of exoplanets to reveal the presence of exomoons, and to measure their mass and distance from the exoplanet.

Until now astronomers have only looked at the changes in the position of a planet as it orbits its star. This has made it difficult to confirm the presence of a moon as these changes can be caused by other phenomena, such as a smaller planet,” said Kipping. “By adopting this new method and looking at variations in a planet’s position and velocity each time it passes in front of its star, we gain far more reliable information and have the ability to detect an Earth-mass moon around a Neptune-mass gas planet.”

Kipping’s work appeared in the December 11th Monthly Notices of the Royal Astronomical Society and could help the search for exomoons that lie within the habitable zone. Of the 300+ exoplanets observed so far, 30 are within the habitable zones of their host stars, but the planets themselves are large gas giants, several times the size of Jupiter. These gas giants are therefore assumed to be hostile for the formation for life (life as we know it in any case) and so have been discounted as habitable exoplanets.

But what if these exoplanets in the habitable zone have Earth-like exomoons orbiting them? Could they be detected? It would appear so.

Prof. Keith Mason, Chief Executive of the Science and Technology Facilities Council (STFC), added, “It’s very exciting that we can now gather so much information about distant moons as well as distant planets. If some of these gas giants found outside our Solar System have moons, like Jupiter and Saturn, there’s a real possibility that some of them could be Earth-like.”

Watch this space for an announcement of the first Earth-like exomoon to be discovered, at the rate of current technological advancement in astronomy, we could be looking at our first Earth-like exoplanet exomoon sooner than we anticipated…

Source: New Scientist, STFC

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Hello! My name is Ian O'Neill and I've been writing for the Universe Today since December 2007. I am a solar physics doctor, but my space interests are wide-ranging. Since becoming a science writer I have been drawn to the more extreme astrophysics concepts (like black hole dynamics), high energy physics (getting excited about the LHC!) and general space colonization efforts. I am also heavily involved with the Mars Homestead project (run by the Mars Foundation), an international organization to advance our settlement concepts on Mars. I also run my own space physics blog: Astroengine.com, be sure to check it out!

Darnell Clayton December 14, 2008 at 9:21 PM

As far as Moons, go, my only question centers on its “day.”

Since its orbit would constantly take it within its parent planet’s shadow, wouldn’t this affect life for the worse?

Also, as far as radiation belts go, both Ganymede and Europa orbit within its radiation belts, although the former is known to host a very strong magnetic field (unfortunately it does not block out all of Jupiter’s radiation).

I think Titan would be a better example, of a large moon that orbits just within its parent worlds magnetic field (despite the fact that it lacks one itself).

dollhopf December 14, 2008 at 10:54 PM

As far as Moons, go, my only question centers on its “day.” Since its orbit would constantly take it within its parent planet’s shadow’

Consider the Moon of the Earth. One could see the Moon in the shadow of the Earth just about hundered times throughout the whole 21st century.

dollhopf December 14, 2008 at 11:04 PM

If there were Exocreatures on an Exomoon of a Exoplanet, would it be easier for them to find Kepler’s Laws than it was for our astronomers? Are celestial mechanics more clearly seen, if one moves around a planet that moves around a star? The imagination makes me dizzy.

Nexus December 15, 2008 at 12:30 AM

It probably wouldn’t take them as long to work out their world isn’t the center of the universe as it took us.

dollhopf December 15, 2008 at 1:33 AM

I don’t know. Even if the moon that spins around the planet, and which itself spins around the sun togehter with its moon, would furthermore spin around its own axis (and not having one side turned toward the planet all the time like our satellite does).

*#-)*

How could they establish a celestial coordinate system so easily? And the seasons on Earth are caused by the change of the inclination during one revolution around the sun. But their seasons would change with every circle around their planet, maybe every month. Thus, what concept of time would they have developed?

dollhopf December 15, 2008 at 1:35 AM

“maybe every month”

I mean OUR month of course. They probably would never have developed a time intervall like a month, which is caused by the observation of our moon.

DJ Barney December 15, 2008 at 2:36 PM

dollhopf …. “exocreatures” .. I like it ! That other word has become sort of … xenophobic ?

I’m scrambling to keep up. Did we get over some kind of “hump” here ? Even if those exoplanets and now exomoons don’t harbour life, the techniques being developed will be invaluable.

Jorge December 15, 2008 at 8:37 AM

In short, I have no answer to Jorge’s question

Oh. Too bad. I was really looking forward to learning something new here.

So, either the experiment doesn’t make sense or there’s something here that’s eluding us. Ian, can’t you squeeze something else about this out of your sources?

law mc December 15, 2008 at 9:43 AM

would radiation really be that big a problem???

the reason we are so susceptible to it is because we are shielded from it, but what if you were always exposed to it, wouldnt life have found ways to mitigate the problems that arise from it. After all, there are bacteria that are very radiation resistant on earth, so i dont think that idea is very strange at all.

ScepticTim December 15, 2008 at 10:48 AM

For those interested, I believe the paper that Ian refers to is arXiv:0810.2243v1 [astro-ph] 13 Oct 2008 “Transit Timing Effects due to an Exomoon” David M. Kipping

Dark Gnat December 15, 2008 at 11:10 AM

Now that some planets have been directly imaged, we could study those images over time to see if the planets themselves wobble. We wouldn’t have to look at the star to determine of the planet has moon(s).

D. Clayton:

If the moon is far enough away, and had an inclination relative to the planet, then it would only pass into the planet’s shadow on occasion. Even if it does regularly pass into the shadow, it be cyclic, and any life forms would have no trouble adapting.

Frank Glover December 15, 2008 at 3:32 PM

“How could they establish a celestial coordinate system so easily? And the seasons on Earth are caused by the change of the inclination during one revolution around the sun. But their seasons would change with every circle around their planet, maybe every month. Thus, what concept of time would they have developed?”

If they watch things long enough, they’ll see patterns and cycles, just as humans did. Perhaps more importantly, such beings might not waste time on the kind of Ptolemaic, planet-centered Universe that western civilization on Earth did…

Beings living on a body directly or indirectly circling one component of a double star might likewise be less inclined to believe the Universe literally revolves around them (unless the companion star was so distant and slow orbiting that it could be mistaken for a background star…)

Yael Dragwyla December 15, 2008 at 10:40 PM

dollhopf: ” ‘maybe every month’ I mean OUR month of course. They probably would never have developed a time intervall like a month, which is caused by the observation of our moon.” From the ever-changing relationship of their primary to their day-star — i.e., from synod to synod, as the primary and the day-star go from conjunction/New to opposition/Full — they could easily derive a time-unit based on the average length of a synod. As for days, that depends on whether they are tidally locked to their primary or not, the length of time it takes them to make one revolution about their primary, and the length of time it takes their primary to orbit their day-star. But they would have a day of some sort for a time-unit, and could peg their month/synod to that, or vice-versa, whichever was shortest.

David Kipping December 16, 2008 at 4:47 AM

Hello all,

I have noticed some confusion about how the method works so I thought I would briefly summarise the idea. I’m very happy to see so many interesting discussions on the topic!

The key point to bear in mind is that we are looking at the wobble of the planet, not the the wobble of the star. Every time a planet transits the star, it affords us information about the position and velocity of the planet. Even if the moon is too small to show up in the transit lightcurve, its effects on the host planet are not.

Both the position and velocity of the planet will be perturbed by the presence of a moon. The spatial perturbation causes the planetary transit to occur slightly earlier or slightly later than expected. The velocity perturbation causes the transit duration to be slightly greater or slightly smaller. Remarkably, these effects can be minutes in magnitude.

The two effects, called transit time variation (TTV) and transit duration variation (TDV) always exhibit a 90 degrees phase difference. In other words, we have a quite unique signature. In addition, the ratio of the two amplitudes allows for a determination of both the moon’s mass and its orbital distance from the host planet.

It’s really fantastic to see so many people interested in exomoons. Who knows how common these exomoons may be and what potential for habitability they possess! Ultimately, the only way for us to answer these questions is to start looking!

All the best and continue to ask questions!
David

Jorge December 16, 2008 at 7:12 AM

Ah! NOW it makes perfect sense! Thanks for the explanation and the best of luck for the experiment.

dollhopf December 17, 2008 at 8:52 AM

DJ Barney told me:

‘”exocreatures” .. I like it ! That other word has become sort of … xenophobic ?’

Yes, but I wonder about when the first manned spaceship will be designed to also carry weapons onboard. Will there once be a need to defend against or dominate over other “earthlings” during missions in space? Or will weapon systems once be integrated into space exploration vehicles or armament be foreseen in view of some “strange” (alien) events?

ROCA February 1, 2009 at 5:00 PM

Hi Ian,
I have a huge interest in extrasolar moons as they solve the problem that an exoplanet inside the habitability zone of a red dwarf star has: tidal locking.
In this scenario a gas giant may be tidal locked to the parent star but the exomoon is not. So the exomoon should have a better climate and may be suitable for life.
A tellurian planet inside the habitability zone is always tidal locked and it is a bad handicap for life as we know it.
Of course a red dwarf creates other problems: emission in infrared and terrible solar storms (solar flares) but this is another theme of discussion.
I translated your great article to Portuguese and here is the link:

Astrônomos começam a caça de exoluas orbitando exoplanetas
http://eternosaprendizes.wordpress.com/2009/02/01/astronomos-comecam-a-caca-de-exoluas-orbitando-exoplanetas/
So, let’s find the exomoons!
Tks
ROCA (Brazil)

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