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All About Space - - Contents -

There are at least 100 bil­lion stars in our gal­axy, and 2 tril­lion galax­ies in the uni­verse. Re­sults from the Ke­pler Space Tele­scope sug­gests that many of these stars have plan­ets. In view of the num­ber of worlds out there, the Fermi para­dox fa­mously asks why we haven’t been con­tacted by other civil­i­sa­tions yet. Per­haps the an­swer is our So­lar Sys­tem is unique in ways that we hadn’t pre­vi­ously con­sid­ered.

The first plan­ets be­yond our So­lar Sys­tem were con­firmed in 1992 by look­ing for stars that wob­bled slightly as they were shifted off cen­tre by the grav­i­ta­tional pull of a planet. This method only de­tects very large plan­ets with very close or­bits, so nat­u­rally it only found star sys­tems quite dif­fer­ent to our own. Then, in 2009 the Ke­pler Space Tele­scope be­gan search­ing for plan­ets by mea­sur­ing the drop in bright­ness as a planet tran­sited in front of the star. This is a much more sen­si­tive method, and in nine years has found over 2,300 con­firmed ex­o­plan­ets. Now the Cal­i­for­nia-Ke­pler Sur­vey has re­fined the or­bital pa­ram­e­ters of al­most 1,000 of the Ke­pler plan­ets us­ing ground-based tele­scopes, and the re­sults an­nounced re­cently are quite trou­bling. It’s not just that most plan­e­tary sys­tems are wildly dif­fer­ent to ours. It’s the fact that they all fol­low a dis­tinct and pre­dictable set of rules, and our So­lar Sys­tem is the odd one out.

Let’s start with the Sun. Our star is a G-type, main se­quence star. This is al­ready un­usual be­cause around 75 per cent of the stars in the gal­axy are M-type red dwarfs, which are smaller and cooler. Even among main se­quence stars ours is one of the bright­est – it out­shines 95 per cent of all the stars in the gal­axy. It is also some­what un­usual in be­ing a loner; more than half of all stars are part of binary sys­tems, where two stars or­bit each other.

When we look at the plan­ets that the Ke­pler Sur­vey has found so far, things get even weirder. The most com­mon type of planet in the gal­axy by far is the ‘su­pert­er­ran’ – a rocky planet up to ten­times Earth masses and 2.5-times Earth's ra­dius.

The next most com­mon is the ‘sub-Nep­tune’ type; a planet with a hy­dro­gen-helium at­mos­phere, but is still less than the mass of Nep­tune. We don’t have a sin­gle ex­am­ple of ei­ther of these planet types in our own sys­tem, and plan­ets that do re­sem­ble our own in size and com­po­si­tion are rare ev­ery­where else.

The dis­crep­ancy be­comes even more ap­par­ent when you con­sider the place­ment of these plan­ets rel­a­tive to their par­ent star. We have four small, rocky, in­ner plan­ets and then four much larger gas giants fur­ther out. But al­most all the ex­o­planet gas giants we have found are well into the ‘hot zone’ of their star (too close for liq­uid wa­ter on the planet sur­face), even though we used to think that

gas giants couldn’t even form that close. In fact, ex­o­plan­ets in gen­eral ap­pear to or­bit their stars much more closely than ours do. Over 93 per cent of all the plan­ets de­tected by Ke­pler are in­side the hot zone of their star. In our So­lar Sys­tem the only planet that close is Mer­cury. “We re­ally have noth­ing in­te­rior to Mer­cury's or­bit,” says Dr Gre­gory Laugh­lin, pro­fes­sor of as­tron­omy and astro­physics at Yale Univer­sity. “There's zilch. There aren't even any as­teroids down there.” Ke­pler-11 on the other hand is a star with five plan­ets or­bit­ing closer than Mer­cury, and this seems to be the norm.

Of course it is much eas­ier to de­tect plan­ets with tight or­bits in the first place be­cause they block more of the star’s light when they tran­sit and they have shorter or­bital pe­ri­ods, so it is eas­ier to spot the cycli­cal pat­tern as they come round each time. So could this skew in the data sim­ply be a con­se­quence of the kind of plan­ets that Ke­pler can de­tect? The lead sci­en­tist of one of the Cal­i­for­nia-Ke­pler Sur­vey stud­ies, Dr Lau­ren Weiss, says not.

“Ke­pler was not sen­si­tive to plan­ets be­yond about 1AU – the Earth-Sun dis­tance. For this rea­son, us­ing Ke­pler alone, we can­not test whether our outer

So­lar Sys­tem is unique. How­ever, the sta­tis­ti­cal prop­er­ties of Ke­pler’s multi-planet sys­tems show us that our in­ner So­lar Sys­tem is un­usual. Most Ke­pler plan­e­tary sys­tems have plan­ets that are very sim­i­lar in size. In con­trast, our ter­res­trial plan­ets have un­usu­ally di­verse sizes. Venus is more than twice the ra­dius of Mer­cury, and Mars is barely half the ra­dius of Earth.”

Most ex­o­plan­ets are just 10 per cent larger or smaller than their im­me­di­ate neigh­bours. To check whether Ke­pler might have missed some plan­ets that would re­sult in more fa­mil­iar-look­ing sys­tems, Dr Weiss tried build­ing imag­i­nary star sys­tems with ran­domly sized plan­ets and then dis­carded all the ones that wouldn’t be de­tected by Ke­pler. “The re­sult… looked noth­ing like the reg­u­lar pat­terns in planet size that we ob­serve, so we re­jected the null hy­poth­e­sis. The sim­i­lar sizes of the plan­ets is as­tro­phys­i­cal, not the re­sult of a de­tec­tion bias.”

An­other strong sta­tis­ti­cal pat­tern that emerged is that planet size in­creases as .you get fur­ther away from the star. This is quite dif­fer­ent from our sys­tem, where the two largest plan­ets, Saturn and Jupiter, sit in the mid­dle. Very large plan­ets that are more than six-times the size of Earth are al­ready quite rare in the gal­axy. Where they oc­cur, it is gen­er­ally in a sys­tem where all the other plan­ets are also large. An­other fac­tor is that 90 per cent of

“Plan­e­tary sys­tems have plan­ets that are very sim­i­lar in size. Our ter­res­trial plan­ets have un­usu­ally di­verse sizes”

Dr Lau­ren Weiss

all the gas giants we have found have or­bits smaller than Mars’. The few that ven­ture fur­ther out are in strongly ec­cen­tric or­bits. Jupiter is par­tic­u­larly pe­cu­liar be­cause it is huge, far away and has an al­most per­fectly cir­cu­lar or­bit. “About one in ev­ery 2,000 stars in our ga­lac­tic neigh­bour­hood is a Sun-Jupiter sys­tem,” says plan­e­tary as­tronomer Dr Sean Ray­mond. “Those are about the odds of be­ing picked if you ap­ply to NASA to be an astro­naut!”

How­ever, it is the quirk­i­ness of our gas giants that could be the key to un­der­stand­ing all the other strange­ness, ac­cord­ing to Dr Weiss. “The role of Jupiter and Saturn was likely very im­por­tant in shap­ing the So­lar Sys­tem. A com­pli­cated dance be­tween Jupiter and Saturn in the early So­lar Sys­tem is of­ten in­voked to ex­plain the anoma­lously small size of Mars. Jupiter and Saturn are also likely re­spon­si­ble for the cur­rent or­bits of Uranus, Nep­tune and the Kuiper Belt. Jupiter also might have helped or hin­dered the de­liv­ery of wa­ter to Earth by way of comets. In­deed, Jupiter and Saturn might be re­spon­si­ble in some not-yet-quan­ti­fied way for the rise of life on Earth.”

Be­fore the first ex­o­plan­ets were dis­cov­ered, the­o­ries of planet for­ma­tion only had to ex­plain our own So­lar Sys­tem. The ear­li­est the­o­ries as­sumed that the plan­ets formed in their cur­rent po­si­tions. "We used to look at the gi­ant plan­ets and think

“Jupiter and Saturn might be re­spon­si­ble in some not-yet-quan­ti­fied way for the rise of life on Earth”

Dr Lau­ren Weiss

those are big, so they never moved," says Dr Kevin Walsh of the SwRI's Plan­e­tary Sci­ence Direc­torate in Colorado. How­ever, com­puter mod­els with these as­sump­tions al­ways pro­duced a Mars that was much too big and an as­ter­oid belt that was much too full. The only way around is to as­sume the gas giants are more mo­bile.

“That an­chor point? It's gone,” says Walsh. Two main com­pet­ing the­o­ries have been put for­ward since then. The Nice model pro­poses that all the large plan­ets formed much closer in and then mi­grated out­ward, trig­ger­ing a bom­bard­ment of pro­to­plan­ets and comets from the outer So­lar Sys­tem. The Grand Tack model goes fur­ther, sug­gest­ing that Jupiter first moved in­ward and then mi­grated out again.

These mod­els go some way in ex­plain­ing Mars and the as­ter­oid belt, but they hit a ma­jor prob­lem when we try to ap­ply them to other plan­e­tary sys­tems be­cause they rely heav­ily on or­bital res­o­nance. This is where one planet makes ex­actly two or­bits for ev­ery one of its neigh­bour, or some other neat ra­tio. These or­bital res­o­nances are com­mon in our So­lar Sys­tem be­cause they are energetically sta­ble, but as soon as we look at other stars, they are nowhere to be found. “The vast ma­jor­ity of the Ke­pler plan­ets are not in mean mo­tion res­o­nances,” says Weiss. “Un­der­stand­ing why has been one of the ma­jor un­solved prob­lems in planet for­ma­tion the­ory over the past few years.”

Ex­o­planet or­bits aren’t ran­dom ac­cord­ing to Dr Weiss. Their or­bits show reg­u­lar-spac­ing pat­terns that are cor­re­lated with the plan­ets’ sizes, but what­ever rule they use to de­ter­mine their po­si­tion, it doesn’t in­volve or­bital res­o­nance. Some­thing about the ini­tial con­di­tions or the move­ments of our gi­ant plan­ets has set the stage dif­fer­ently. “Our So­lar Sys­tem is a bit of a weirdo,” says Weiss.

There’s a big gap be­tween odd and unique, though, and some as­tronomers are not con­vinced we’re all that spe­cial. "I would be very sur­prised if the So­lar Sys­tem were re­ally strange," says Jack Lis­sauer, a plan­e­tary sci­en­tist at NASA Ames Re­search Center in Cal­i­for­nia. "There are so many stars out there. Even if it's only one per cent, it's still not re­ally rare."

When the Tran­sit­ing Ex­o­planet Sur­vey Satel­lite (TESS) be­gins its two-year sur­vey of the en­tire sky this year, it will cover 400 times as much area as Ke­pler and look for plan­ets around more than 200,000 stars, but even TESS won’t be able to see a Earth-sized planet in an Earth or­bit around a star like our Sun. Pre­dict­ing how rare Earth re­ally is will still rely on sci­en­tists fully un­der­stand­ing how plan­ets form and evolve.

free gift

Sun

About 99.86 per cent of the mass in the So­lar Sys­tem is present in the Sun. It formed about 4.6 bil­lion years ago and is about half­way through the main-se­quence stage of its life. The Sun is cur­rently grow­ing brighter. Earth

Earth is strange in lots of ways. It is the dens­est planet in our So­lar Sys­tem and the only one with liq­uid wa­ter on its sur­face. Our only satel­lite, the Moon, is the largest in re­la­tion to the size of the planet. It may have been formed by a head-on-col­li­sion from a Mars-sized ob­ject, known as Theia, 4.5 bil­lion years ago.

The Moon Venus

Venus has the slow­est ro­ta­tion of any of the plan­ets, so it is al­most per­fectly spher­i­cal. A day on Venus lasts longer than its year! It has no moons now, but bil­lions of years ago it may have had at least one that has since been de­stroyed. Mer­cury

Mer­cury is the clos­est planet to the Sun, but in ga­lac­tic terms its or­bit is still quite large. A year on Mer­cury lasts 88 Earth days, whereas other stars typ­i­cally have in­ner plan­ets that or­bit in less than a week.

Pho­bos Main as­ter­oid belt

Although there are mil­lions of as­teroids in the main belt, their to­tal mass is only about 4 per cent of the Moon, and a third of this is ac­counted for by the dwarf planet Ceres. Grav­i­ta­tional dis­rup­tion from Jupiter cleared out most of the early as­teroids.

Deimos Mars

The small size of Mars is quite odd. It is dra­mat­i­cally smaller than ei­ther of its neigh­bours and sim­ple mod­els of planet for­ma­tion tend to pre­dict a much larger planet.

Cal­listo Europa Amalthea

Io Ganymede

Hi­malia

Saturn

Con­sist­ing al­most en­tirely of hy­dro­gen and helium, Saturn is the only planet with an av­er­age den­sity lower than wa­ter. Most of Saturn’s hy­dro­gen is liq­uid, with a metal­lic, rocky core sur­rounded by metal­lic hy­dro­gen. Saturn has the most prom­i­nent ring sys­tem of any planet and an enor­mous hexag­o­nal storm at its north pole. Jupiter

If you com­bined ev­ery other planet in the So­lar Sys­tem, Jupiter would still be 2.5times more mas­sive. This gi­ant com­prises, by mass, 89 per cent hy­dro­gen, 10 per cent helium and small amounts of meth­ane and am­mo­nia. Jupiter and

Saturn are locked in a 2:5 or­bital res­o­nance.

Hype­r­ion Rhea Tethys Mi­mas Epimetheus

Janus Ence­ladus Dione Ti­tan Iape­tus Phoebe Oberon Um­briel Mi­randa

Por­tia Puck Ariel Ti­ta­nia Sy­co­rax Uranus

The light­est gi­ant planet, and with Nep­tune, it was given a sep­a­rate clas­si­fi­ca­tion in the 1990s an ice gi­ant. It has a small iron/nickel core with a wa­ter/am­mo­nia/meth­ane-ice man­tle, and a low­den­sity at­mos­phere of hy­dro­gen and helium. Uranus has a much lower in­ter­nal tem­per­a­ture than the other gi­ant plan­ets. This means that although Nep­tune is the fur­thest away, Uranus is the cold­est planet in the So­lar Sys­tem. Nep­tune

Nep­tune is the dens­est of the gi­ant plan­ets. Its iron/nickel core alone is more mas­sive than Earth. Like Uranus, Nep­tune is re­ferred to as an ice gi­ant, even though its man­tle is a hot, su­per­pres­surised mix of wa­ter and am­mo­nia. Tri­ton Larissa De­spina Galatea Pro­teus Nereid

Charon Pluto

Although it is one of the largest ob­jects in the Kuiper Belt, we now know that there are oth­ers in the same size class. Icy Pluto was the for­mer ninth planet in our So­lar Sys­tem, but in 2006 it was for­mally down­graded to the new class of dwarf planet.

Most ex­o­plan­ets or­bit much closer to their star than the or­bit of Mer­cury

The Ke­pler Space Tele­scope was launched in March 2009 in a Delta II rocket

Artist’s im­pres­sion of the Ke­pler Space Tele­scope, su­per­im­posed over a plan­e­tary tran­sit (not to scale)

TESS will soon ex­tend the search for ex­o­plan­ets across the en­tire sky

Mars is un­usu­ally small, which has con­trib­uted to the loss of its at­mos­phere

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