"Long before it's in the papers"
January 27, 2015


When can a moon harbor life? Scientists get down to nitty-gritty

Jan. 10, 2013
Courtesy of Leibniz-Institut für Astrophysik 
Potsdam, University of Oxford
and World Science staff

With the no­tion of life on moons of dis­tant so­lar systems draw­ing grow­ing in­ter­est from as­tro­no­mers, some are start­ing to re­search ex­actly what might make such a moon hab­it­a­ble. 

The an­swer turns out to be more com­plex than you might ex­pect.

Just as with plan­ets, moons have their own “hab­it­a­ble zones,” ar­eas in which they can re­side with­out los­ing the chance to have liq­uid wa­ter or life, said Ro­ry Barnes of the Uni­vers­ity of Wash­ing­ton and the NASA As­tro­bi­ol­o­gy In­sti­tute.

Artist’s con­ception of Earth-like moons orb­i­ting a gi­ant ga­seous plan­et. (Cred­it: R. Hell­er, AIP)

But for moons the hab­it­a­ble zone is “a lit­tle dif­fer­ent” and in some ways more re­strict­ed, added Barnes, co-author of a new study on the sub­ject. For in­stance, a moon’s hab­it­a­ble zone de­pends a good deal on the dis­tance to its host plan­et, a fac­tor un­likely to greatly af­fect the plan­et’s own liv­abil­ity.

As­tro­no­mers are in­trigued by the idea of life on moons or­bit­ing plan­ets out­side our so­lar sys­tem, called “ex­o­moons.” Sev­er­al fac­tors make the pos­si­bil­ity en­tic­ing. One is that a sin­gle plan­et can host sev­er­al moons, any of which with luck might be hab­it­a­ble—e­ven if the plan­et it­self is not, which is con­sid­ered a com­mon sce­nar­i­o con­sid­er­ing big plan­ets are usu­ally made of gas.

“Jupiter has several large, water-rich moons. Ima­gine drag­ging that sys­tem into the com­fort­ably warm re­gion where the Earth is. If such a plan­et had Earth-size moons, we'd see... worlds with rivers, lakes and all sorts of habi­tats—a surprising sce­nario that might just be com­mon,” said astronomer Chris Lin­tott of Ox­ford Uni­ver­sity. He is a col­la­bor­ator in a pro­ject in which citi­zen volun­teers can help find plan­ets using a website, Plan­et­hun­ters.org.

There are about 20 known plan­ets in plan­etary “ha­bit­able zones,” and of these worlds might have moons, Lint­ott added. Just this week, Plan­et Hunt­ers vol­un­teers added one to that list, a con­firmed plan­et dubbed PH2 b, he said. The find­ing has been sub­mitted to The Astro­phys­i­cal Jour­nal and posted online.

A prob­lem with finding life on moons is that due to their rel­a­tively small size, they’re hard to de­tect. And they gen­er­ally can’t re­tain an at­mos­phere if they’re much smaller than Earth, said Barnes and his co-author, René Hell­er of German­y’s Leib­niz In­sti­tute for As­t­ro­phys­ics Pots­dam. Our so­lar sys­tem’s larg­est moon, Ju­pi­ter’s Gan­ymede, weighs a pu­ny one-fortieth of Earth, rais­ing the ques­tion of wheth­er big enough moons even ex­ist—but noth­ing rules out the pos­si­bil­ity, Hell­er and Barnes said.

The fi­ne pre­ci­sion of NASA’s Kep­ler space tel­e­scope now makes the de­tection of a Mars-to-Earth sized moon pos­si­ble, in­deed im­mi­nent, the two re­search­ers added. Their anal­y­sis on ex­o­moon hab­it­abl­ity ap­pears in the Jan­u­ary is­sue of the jour­nal As­tro­bi­ol­o­gy and is al­so on­line he­re.

The cli­mate on ex­o­moons will likely dif­fer from those on the plan­ets they are or­bit­ing, or “exoplan­ets,” they wrote. For one thing, moons are typ­ic­ally tid­ally locked to their host plan­et. This means that, as with our moon, the same side of the moon al­ways faces the plan­et. 

Moons would al­so have two sources of light: the star and the plan­et. But the plan­et would some­times block out their “sun.” This could cre­ate dras­tic and cli­mate-altering eclipses, es­pe­cially con­sid­er­ing that the block­ing ob­ject might be an im­mense plan­et. “An ob­serv­er stand­ing on the sur­face of such an ex­o­moon would ex­pe­ri­ence day and night in a to­tally dif­ferent way than we do on Earth,” ex­plained Hell­er. “Stel­lar eclipses could lead to sud­den to­tal dark­ness at noon.”

An­oth­er dis­rup­tive in­flu­ence for a moon is tid­al heat­ing, Hell­er and Barnes said. Our moon cre­ates rel­a­tively mild tid­al ef­fects on Earth, but if there were a much heav­i­er and clos­er body in our sky, that could be a dif­ferent sto­ry: the tides might be strong enough to lit­er­ally bend the ground be­neath our feet, cre­at­ing fric­tion that heats our whole world. Moons that or­bit their host plan­et too closely will un­dergo strong tid­al heat­ing that would boil away sur­face wa­ter and rend­er them un­inhab­it­a­ble, Barnes and Hell­er ex­plained.

That’s why a moon can’t be more than so close to its host plan­et with­out be­com­ing un­live­a­ble, they added. That min­i­mum dis­tance sep­a­rat­ing the two bod­ies, they have dubbed the “hab­it­a­ble edge”—de­ter­mined by a com­pli­cat­ed for­mu­la that de­pends on fac­tors in­clud­ing the plan­et’s mass, the moon’s re­flec­ti­vity and the cir­cu­lar­ity of its or­bit. This con­cept will al­low fu­ture as­tro­no­mers to eval­u­ate the hab­it­abil­ity of dis­tant moons, the re­search­ers pro­pose.

In ad­di­tion to these con­di­tions, moons face an­oth­er in­flu­ence on hab­it­abil­ity that al­so af­fects their host plan­ets: the dis­tance to the host star of both bod­ies, and the prop­er­ties of that star. These fac­tors bear on wheth­er the moon-plan­et sys­tem has warmth suit­a­ble to sup­port liquid wa­ter.

* * *

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With the notion of life on far-off moons drawing increasing interest from astronomers, some are starting to research exactly what might make such a world habitable. The answer turns out to be more complex than you might expect. Just as with planets, moons have their own “habitable zones,” areas in which they can reside without losing the chance to have liquid water or life, said Rory Barnes of the University of Washington and the NASA Astrobiology Institute, co-author of a new study on the subject. But for moons the habitable zone is “a little different” and in some ways more restricted, he added. For instance, a moon’s habitable zone depends a good deal on the distance to its host planet, a factor unlikely to greatly affect the planet’s own livability. Astronomers are intrigued by the idea of life on moons orbiting planets outside our solar system, called “exomoons.” Several factors make the possibility enticing. One is that a single planet can host several moons, any of which with luck might be habitable—even if the planet itself is not, which is considered a common scenario considering big planets are usually made of gas. But a problem is that exomoons, due to their relatively small size, are hard to detect. And they generally can’t retain an atmosphere if they’re much smaller than Earth, said Barnes and his co-author, René Heller of Germany’s Leibniz Institute for Astrophysics Potsdam. Our solar system’s largest moon, Jupiter’s Ganymede, weighs a puny one-fortieth of Earth, raising the question of whether big enough moons even exist—but nothing rules out the possibility, Heller and Barnes said. The fine precision of NASA’s Kepler space telescope now makes the detection of a Mars-to-Earth sized moon possible, indeed imminent, the two researchers added. Their analysis on exomoon habitablity appears in the January issue of the journal Astrobiology and is also published online here. The climate on exomoons will likely differ from those on the planets they are orbiting, or “exoplanets,” moons are typically tidally locked to their host planet, they wrote. This means that, as with our moon, one side of the moon permanently faces the planet. Moons would also have two sources of light—that from the star and the planet they orbit. But the planet would sometimes block out their “sun.” This could create drastic and climate-altering eclipses, especially considering that the blocking object might be an immense planet. “An observer standing on the surface of such an exomoon would experience day and night in a totally different way than we do on Earth,” explained Heller. “Stellar eclipses could lead to sudden total darkness at noon.” Another disruptive influence for a moon is tidal heating, Heller and Barnes said. Our moon creates relatively mild tidal effects on Earth, but if there were a much heavier and closer body in our sky, that could be a different story: the tides might be strong enough to literally bend the ground beneath our feet, creating friction that heats our whole world. Moons that orbit their host planet too closely will undergo strong tidal heating that would boil away surface water and render them uninhabitable, Barnes and Heller explained. That’s why a moon can’t be more than so close to its host planet without becoming unliveable, they added. That minimum distance separating the two bodies, they have dubbed the “habitable edge”—determined by a complicated formula that depends on factors including the planet’s mass, the moon’s reflectivity and the circularity of its orbit. This concept will allow future astronomers to evaluate the habitability of extrasolar moons, the researchers propose. In addition to these conditions, moons face another influence on habitability that also affects their host planets: the distance to the host star of both bodies, and the properties of that star. These factors bear on whether the moon-planet system has warmth suitable to support liqud water. About 850 planets outside the solar system are known. Most are believed to be sterile gas giants, similar to Jupiter. Only a handful are thought to have a solid surface and orbit their host stars in the habitable zone.