Science Moves Closer to Discovering Exo-Moons

2015 may end up being the “Year of the Exo-Moon”.

It is not an original thought by me but rather the opinon of Exoplanet hunter and
characterizer extraordinaire Sara Seager

Exomoons are fascinating because they present another possibly habitable environment for life outside our solar system. We know of tons of “Warm
Jupiters”, massive planets like Jupiter but orbiting within the habitable zone of their star. In fact between Kepler and other exoplanet research
programs we know of around 1,000 Warm Jupiters (aka Warm Jovians):

While these planets themselves are not habitable, they may host large moons like Jupiter and Saturn do in our Solar System, however unlike the cold,
icy moons of Jupiter and Saturn these exomoons would be warmed by their star in a way similar to the way the Earth is warmed by our Sun since the
planets these moons would orbit would be in the “goldilocks zone”.

“Sunset” on a habitable exomoon

In fact one of the first exoplanets ever discovered, Upsilon Andromedae d, is a large gas giant which orbits in the habitable zone of a star in our
nearby neighborhood 44 light years away. It was discovered by Geoffrey Marcy of the California Planet Search at Lick Observatory in 1999. He considers
it his favorite planet discovery due to this possibility of a habitable moon, even though he has discovered more planets than anyone else on Earth (70
of the first 100 exoplanets were found by Marcy).

ANIMATION: I simulated the unique sunrise/sunset cycle on a habitable moon circling Upsilon Andromedae d

Habitable exomoons also feature heavily in science fiction because the sky of such a moon is visually striking, dominated by the gas giant it orbits,
sometimes with a ring system.

As a result, such moons have featured in Hollywood blockbusters like Star Wars, Avatar and the Star Trek franchise such as Pandora and Endor, pictured

But far from being only figments of sci-fi, exomoons are almost certain to exist and it would be shocking if we did not find them.

Planetary formation models produce exomoons in spades so the next step has always been to look for them. Sounds easy enough right?

In actuality they have remained mostly undetectable until the launch of NASA’s Kepler Mission and even with Kepler data, the signature or “signal” of
an exomoon in a Kepler lightcurve is miniscule, almost close to the random noise so they are very difficult to tease out.

That has not stopped people from trying to develop techniques where such moons pop out of the noise, and the most notable of these researchers is Dr.
David Kipping of Harvard University’s HEK (Hunting Exomoons with Kepler) project.

Below is an interesting lecture by him which is not too technical (and also references sci-fi) from his “Life as a Planetary Phenomena” class at

Recently Dr. Kipping has published a paper which puts some limits on the size of moons detectable using Kepler data around various size exoplanets.
This is a key step in the road to the first discovered and confirmed exomoon of any type, habitable or not.

This research is fresh, having just been posted a few days ago by the Astrophysical Journal.

The paper looks closely 41 Kepler Objects of Interest (KOIs), these are Kepler planet candidates, 90% of which will be confirmed.

The research brings the total number of KOIs examined by HEK up to 57 and shows that the process is beginning to move beyond just computer
simulations and assumption heavy theory to actual Kepler data.

The number one aim of the paper?

To determine how small a moon could be detected around each of these planet candidates given the kind of predicted tell-tale signs that would reveal
an exomoon’s existence.

GRAPHIC: After looking at around 60 exoplanets for moons, the HEK team have defined limits of detectability for each world. This shows an
ability to detect even the smallest moons capable of sustaining an Earth-like atmosphere (“Mini-Earths”) for 1 in 4 cases studied. Though a
confirmed discovery has not been made, the detailed survey of 60 planets spanning several years reveals what can be done with current technology. –
The Hunt for Exomoons with Kepler (HEK) Project.

So, basically, this shows the value of the “null detection” in science.

If an exoplanet that is closely examined does not turn up an exomoon that leads to a statement of how massive a moon has been excluded by the current
data, which means the team is learning much about the sensitivity of its methods. From the paper:

… based on empirical sensitivity limits, we show for the first time that the HEK project is sensitive to even the smallest moons capable of
being Earthlike for 1 in 4 cases (after accounting for false-positives). In terms of planet-mass ratios, we find even that the Earth-Moon mass-ratio
is detectable for 1 in 8 of cases, posing a challenge but not an insurmountable barrier. Mass ratios of ∼ 10−4, such as that of the Galilean
satellites, have never been achieved. However, if Galilean-like satellites reside around lower-mass planets than Jupiter, of order ∼ 20 M⊕, then
we do find sensitivity, as demonstrated by the limit of 1.7 Ganymede masses achieved for Kepler-10c.

Which is good news, because the team can now make factual statements about the actual mass of a detectable exomoon. In 1 of 3 planets surveyed, an
exomoon with Earth’s mass is detectable. Kipping believes that we can move down to the smallest moon thought capable of supporting an Earth-like
atmosphere and still detect it in 1 of 4 of the cases studied. No exomoons have yet been detected but we are learning just what our capabilities are.

According to Kipping:

“Here we report on our null results and the first estimate of empirical sensitivities. Ultimately, we would like to actually discover a clear
signal and are pursuing some interesting candidates we hope will pan out. Either way though, I like to recall what the Nobel Prize winning American
physicist Richard Feynman said about science: ‘Nature is there and she’s going to come out the way she is, and therefore when we go to investigate
it we shouldn’t pre-decide what it is we’re trying to do except to find out more about it’.”

Image: The Moon has about 1% the mass of the Earth posing a challenge for the HEK team, since such configurations are detectable for 1 in
8 planets surveyed. The much larger Pluto-Charon mass-ratio of 11.6% is much more detectable. – Hunt for Exomoons with Kepler (HEK)

While no exomoons were detected in the 41 KOIs surveyed in the study, four, KOI-0092.01, KOI-0458.01, KOI-0722.01 and KOI-1808.01, showed up as false
positives for an exomoon. Stellar activity is a likely cause of the false positives nevertheless this demonstrates that such moons, if they are out
there are detectable and that such an exciting discovery could come as soon as this year.

Paper: The Hunt for Exomoons with Kepler (HEK): V. A Survey of 41 Planetary Candidates for Exomoons
Kipping, et al.

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