From Stars to Planets: Objects in Space and Habitability Potential

Planetary system development (artist’s impression)

Only about 4% of the total mass of the Uni­verse is made of atoms or ions, and thus rep­re­sent­ed by chem­i­cal ele­ments. Hydro­gen is the most abun­dant ele­ment, mak­ing up 75% of nor­mal mat­ter by mass and over 90% by num­ber of atoms. It is found in great quan­ti­ties in stars and gas giant plan­ets, as well as in the inter­stel­lar medi­um.

Most of the mass of the Uni­verse, how­ev­er, is not in the form of chem­i­cal-ele­ment type mat­ter. It is pos­tu­lat­ed to occur as forms of mass such as dark mat­ter and dark ener­gy.

The first stars that formed after the Big Bang, pro­vid­ed the Uni­verse with the first ele­ments heav­ier than heli­um (‘met­als’), which were incor­po­rat­ed into low-mass stars that have sur­vived to the present. For exam­ple, eight stars in the old­est glob­u­lar clus­ter in our Galaxy, NGC 6522, were found to have sur­face abun­dances con­sis­tent with the gas from which they formed being enriched by mas­sive stars dur­ing the ear­ly phas­es of the seed­ing of heavy ele­ments.

Recent study sug­gests that the very first star might have been born much ear­lier than pre­vi­ous­ly thought, when the Uni­verse was only 30 mil­lion years old. Inter­est­ing ques­tion about the ear­li­est stars is, were the­se mon­sters blue-vio­let and lumi­nous or dark and glow­ing infrared? Could the ear­ly Uni­verse pos­si­bly have had both types? In a place that had not yet wit­nessed chem­i­cal and mag­net­ic stel­lar feed­back, the for­ma­tion of the first stars is a well-defined prob­lem for the­o­rists.

Of 118 known only 94 ele­ments occur nat­u­ral­ly. Most of the ele­ments heav­ier than heli­um are syn­the­sized in stars when lighter nuclei fuse to make heav­ier nuclei. The process is known as nucle­osyn­the­sis. The rest is pro­duced in super­novae and oth­er vio­lent cos­mic events. The mate­ri­al in our sun (and solar sys­tem) has been cycled through at least sev­er­al stars.

Stellar Cycle. Image credit: Seth Stein
Stel­lar Cycle. Image cred­it: Seth Stein

The more rounds of star birth and death there have been, the larg­er the ratio of car­bon to oxy­gen. Towards the cen­ter of our Galaxy, in the regions of more evolved stel­lar pop­u­la­tion, you’d expect to find more car­bon worlds than sil­i­cate plan­ets like Earth. Sim­i­lar­ly, over time, the gas clouds across the whole galaxy are get­ting pro­gres­sive­ly more car­bon-rich, and in a few bil­lion years, most of new plan­ets might turn out to be car­bon worlds.

As the Uni­verse ages, one might ask, is there a pos­si­bil­i­ty of appear­ance of yet unknown ele­ments with changes in stel­lar pop­u­la­tions? Well, no. In our present-day Uni­verse nat­u­ral­ly occur­ring exotic Unob­ta­ni­um is unlike­ly. Even if tech­nol­o­gy is involved, the exist­ing peri­od­ic table plus some anti­mat­ter (anti-hydro­gen any­one?) is all we get. Per­haps, in anoth­er uni­verse with slight­ly dif­fer­ent laws of physics, allow­ing such meta­mor­pho­sis? Who knows?

Ele­men­tal abun­dances in the Uni­verse drop off expo­nen­tial­ly with increas­ing atom­ic num­ber (Z) up to Z ~ 60; there­after remain almost con­stant. Even-Z ele­ments are more abun­dant than their odd-Z neigh­bors. Li, Be & B show marked deple­tion rel­a­tive to both high­er and low­er-Z ele­ments.

Element abundance curve and production processes
Ele­ment abun­dance curve and pro­duc­tion process­es

Abun­dance may be var­i­ous­ly mea­sured by the mass-frac­tion (the same as weight frac­tion), or mole-frac­tion (frac­tion of atoms by numer­i­cal count, or some­times frac­tion of mol­e­cules in gas­es), or by vol­ume-frac­tion. Mea­sure­ment by vol­ume-frac­tion is a com­mon abun­dance mea­sure in mixed gas­es such as plan­e­tary atmos­pheres.

Assum­ing that life can only evolve on the basis of car­bon com­pounds, there are sev­er­al extreme­ly impor­tant ele­ments, required for life.

Important elements for life
Impor­tant ele­ments for life

By mass, human cells con­sist of 65–90% water and 99% of the mass of the human body is made up of the six ele­ments, which are oxy­gen (65%), car­bon (18%), hydro­gen (10%), nitro­gen (3%), cal­ci­um (1.5%), and phos­pho­rus (1.2%). Need­less to say, life is built from the most abun­dant stuff, avail­able at the spot. How­ev­er, note that Earth is sil­i­con-rich and car­bon-poor, e.g. has more sil­i­con (27.69%, sec­ond after oxy­gen by mass) than car­bon (only 0.094%) in its crust, and life still had cho­sen car­bon as its base; that’s prob­a­bly uni­ver­sal.

Also, hydro­gen may be the most abun­dant ele­ment in the Uni­verse, but oxy­gen has an impor­tance that is dis­pro­por­tion­ate to its abun­dance, since it plays a crit­i­cal role in so many fun­da­men­tal plan­e­tary sys­tem process­es. The very nature of the ter­res­tri­al plan­ets in our own Solar Sys­tem would be much dif­fer­ent had the oxygen/carbon ratio in the ear­ly solar neb­u­la been some­what low­er than it was, because ele­ments such as cal­ci­um, iron and tita­ni­um would have been locked up dur­ing con­den­sa­tion as car­bides, sul­fides and nitrides and even (in the case of sil­i­con) part­ly as met­als rather than sil­i­cates and oxides.

Play­ing chess on the peri­od­ic table 

Extra­so­lar plan­ets often dif­fer tremen­dous­ly from the worlds in our solar sys­tem. Rather than assume plan­ets around oth­er stars are scaled-up or scaled-down ver­sions of the plan­ets in the Solar Sys­tem, it is nec­es­sary to con­sid­er all types of plan­ets that might be pos­si­ble, given what is known about the com­po­si­tion of pro­to­plan­e­tary discs.

Plan­ets can have a wide range of sizes and mass­es but plan­ets made of the same mate­ri­al will have the same den­si­ty regard­less of their size and mass. For exam­ple, a huge, mas­sive plan­et can have the same den­si­ty as a small, low-mass plan­et if they are made of the same mate­ri­al. No mat­ter what mate­ri­al a plan­et is made of, the mass/diameter rela­tion­ship fol­lows a sim­i­lar pat­tern. All solids com­press in a sim­i­lar way because of the struc­ture. If you squeeze a rock, noth­ing much hap­pens until you reach some crit­i­cal pres­sure, then it breaks. The same goes for plan­ets, but their oper­at­ing pres­sure is com­po­si­tion-depen­dent.

In 2007 a team of sci­en­tists have cre­at­ed mod­els for 14 dif­fer­ent types of solid plan­ets that might exist in our Galaxy. The 14 types have var­i­ous com­po­si­tions, and the team cal­cu­lat­ed how large each plan­et would be for a given mass. Some are pure water ice, car­bon, iron, sil­i­cate, car­bon monox­ide, and sil­i­con car­bide; oth­ers are mix­tures of the­se var­i­ous com­pounds.

Comparison of sizes of planets with different compositions. Credit: Marc Kuchner/NASA GSFC
Com­par­ison of sizes of plan­ets with dif­fer­ent com­po­si­tions. Cred­it: Marc Kuchner/NASA GSFC

Plan­ets fall into dif­fer­ent class­es, with our solar Sys­tem being rep­re­sent­ed only by two of them – rocky ter­res­tri­als and gas giants. But rocky plan­ets almost as big as Uranus seem far more com­mon in the Uni­verse than sus­pect­ed. The­se Super-Earths, now an offi­cial­ly defined plan­et class, have mass­es in the range from Earth to Uranus, exact­ly the range that is miss­ing from our Solar Sys­tem.

Ter­res­tri­al plan­ets have over­all den­si­ties around 4–5 g/cm^3 with sil­i­cate rocks on the sur­face. Sil­i­cate rock has a den­si­ty ~ 3 g/cm^3 and iron has a den­si­ty ~ 8 g/cm^3. Ocean plan­ets will prob­a­bly be some­where between 2 and 4 g/cm^3 (depend­ing on inte­ri­or mate­ri­als), and gaseous plan­ets are thought to be hold­ing at ~ 0.6 – 2 g/cm^3 (check out Sat­urn, which is less dense than water!) but there were some sur­pris­es as well. In 2006 a plan­et with extreme­ly low den­si­ty was dis­cov­ered. This extra­so­lar plan­et, TrES-4b is 70% larg­er but actu­al­ly less mas­sive than Jupiter. With the large size, and low­er mass, the plan­et has a low den­si­ty of only 0.22 grams per cubic cen­time­ter, and is prob­a­bly com­posed of hydro­gen and heli­um. And this isn’t the sole case. Cur­rent­ly there is a whole group of such dif­fuse “puffy” plan­ets. The­se gas giants with a large radius and very low den­si­ty some­times are called “hot Sat­urns”, due to their sim­i­lar den­si­ty to Sat­urn.

Ter­res­tri­al planet’s bulk den­si­ty can be approx­i­mat­ed as rho_­plan­et = rho_earth*(Mplanet/Mearth)^0.2, where Mplan­et and Mearth are body mass­es and rho_earth is the mean den­si­ty of Earth. The uncom­pressed den­si­ty of such plan­et would be earth­like, and so will the com­po­si­tion. More mas­sive objects have stronger grav­i­ties. As a result, more mas­sive objects get more com­pressed than less mas­sive objects. This com­pres­sion means that the bulk den­si­ty of the object will be greater. Uncom­pressed den­si­ty is free of mass depen­dence.

Now sum­ming up all things pre­vi­ous­ly said, com­po­si­tion along with plan­e­tary mass plays a vital role for hab­it­abil­i­ty poten­tial, allow­ing or restrict­ing such things as atmos­phere, mag­net­ic field and plate tec­ton­ics to cre­ate ocean basins, with vol­can­ism dri­ving car­bon­ate-sil­i­cate cycle, and, even­tu­al­ly, life.

Size of a hab­it­able plan­et

Very small ter­res­tri­al plan­ets (around or less than 0.3 Earth mass­es) are less like­ly to retain the sub­stan­tial atmos­pheres (because of low grav­i­ty) and ongo­ing tec­ton­ic activ­i­ty prob­a­bly required to sup­port life. By “ongo­ing” I mean sev­er­al bil­lions of years. The same is with the upper mass lim­its. Very large ter­res­tri­al plan­ets (per­haps, larg­er than 2 Earth mass­es) might have some prob­lems with becom­ing geo­log­i­cal­ly dead faster than life might evolve there, or being hyper­ac­tive, reshap­ing their sur­face quite fast. I will talk about plate tec­ton­ics (and maps!) in details some­where in the fol­low­ing posts, so I’m not going to focus on that here.

Then, there is anoth­er issue with the size. In 2010, mod­el sim­u­la­tions of rocky super-Earths between 2 and 10 Earth-mass­es showed that high pres­sures could keep their cores solid instead of molten, which would pre­vent a mag­net­ic field from form­ing to pro­tect devel­op­ing sur­face life from stel­lar radi­a­tion. Oth­er sci­en­tists argue that the inte­ri­ors of super-Earth may still get hot enough to melt their iron cores despite the pres­sure due to oth­er fac­tors not yet con­sid­ered by the mod­el sim­u­la­tion.

Mag­net­ic shield­ing

Plan­ets in HZs are exposed to stel­lar winds; the closer to the star, the denser the wind. With­out a mag­net­ic field gen­er­at­ed by a rotat­ing molten metal­lic core, the atmos­phere of such a plan­et would also face pro­gres­sive ero­sion by the stel­lar wind of its host star. The plan­et, even if rotat­ing slow­ly (as in tidal­ly locked), can have strong mag­net­ic shield­ing given the suit­able mass, size, chem­i­cal com­po­si­tion and effec­tive con­vec­tion in its inte­ri­ors. This was very well shown in the mod­el by Bar­nes et al, 2010.

Magnetic moment strength. The values are lower limits to the expected magnetic moment strengths. Image credit: Barnes et al. 2010
Mag­net­ic moment strength. The val­ues are low­er lim­its to the expect­ed mag­net­ic moment strengths. Image cred­it: Bar­nes et al. 2010

The image rep­re­sents mag­net­ic moment mod­el esti­mates for plan­ets up to 12 Earth mass­es, a pure iron core, and perovskite/ferropericlase mantle com­po­si­tions. The col­or scale on the right cor­re­sponds to mag­net­ic moment val­ues between 0 and 80 times that of the Earth. The region below the col­ored squares cor­re­sponds to plan­ets made out of core mate­ri­als denser than iron, while the region above cor­re­sponds to plan­ets with radii too large, and there­fore too low den­si­ty, to have a core capa­ble of gen­er­at­ing a mag­net­ic field.

How­ev­er, note that plan­ets under extreme con­di­tions (i.e. high­ly inho­mo­ge­neous heat­ing or very strong stel­lar winds), will have their mag­net­ic fields affect­ed.

All this cor­re­sponds pret­ty well with the pre­vi­ous mod­el.

Mass/Diameter/Composition Relation. Credit: Marc Kuchner/NASA GSFC
Mass/Diameter/Composition Rela­tion. Cred­it: Marc Kuchner/NASA GSFC

Con­sid­er now a hab­it­able ocean plan­et. In order to pro­tect the ocean from evap­o­rat­ing you need an atmos­phere. A mag­net­ic field is nec­es­sary to pre­vent the atmos­phere from even­tu­al­ly being stripped away by stel­lar wind. Also, a plan­et too close to its star or a giant plan­et (in case of a satel­lite) can be a sub­ject to seri­ous tidal heat­ing. So give a thought about planet’s inte­ri­ors and how they work.

Jeno Marz
JENO MARZ is a science fiction writer from Latvia, Northern Europe, with background in electronics engineering and computer science. She is the author of two serial novels, Falaha’s Journey: A Spacegirl’s Account in Three Movements and Falaha’s Journey into Pleasure. Marz is current at work on a new SF trilogy. All her fiction is aimed at an adult audience.

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