The planetary definition data resides in the data/universe/planets.xml and
the XML files inside the data/universe/planets/ folder.

Each of those files are a simple XML file with <planet> entries. The tag
names are case-sensitive. The files are read in a consistent order:
First data/universe/planets.xml, then the files in data/universe/planets/
subfolder, sorted case-insensitively. Information read later in the process
overwrites information read earlier.

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<planets>
    <planet>...</planet>
</planets>

Inside the <planet> tag, the following three tags are required:

<name>(string)</name> - the name of the planet
<xcood>(number)</xcood> - X-coordinate on the map, in light years
<ycood>(number)</ycood> - Y-coordinate, also in light years

The remaining information is optional:

<id>(string)</id>

Internal ID of the planet. If not set, equals to the name.
Used to allow multiple planets to share the same name.

If the same ID (or failing that, name) is encountered multiple
times, the later values overwrite the earlier defined values.

<shortName>(string)</shortName>

Short variant of the name, for display purposes.

<sysPos>(integer)</sysPos>

The planet's orbital position in the system.

<spectralType>(string)</spectralType>

The star's spectral type. MekHQ can parse most simple types like
"G2V", "sdM4.5", "O3IIIne", "B5V(e)+DQ6.2" and so on. If set,
the following three values are ignored.

Spectral types starting with "Q" are non-stellar points in space.
In particular, "QB" are black holes, "QN" neutron stars and
"QP" pulsars.

If missing, the spectral type gets generated from the following
values (spectralClass, subtype and luminosity). If those are
also missing, a procedurally generated "habitable" main sequence
dwarf star is set.

<spectralClass>(character)</spectralClass>

One-character description of the spectral class. Valid values
are "O", "B", "A", "F", "G", "K", "M", "L", "T" and "Y"

Ignored if spectral type is set. If only the spectral class is
set, subtype is generated randomly and luminosity is set to "V".

Also ignored if the luminosity is set to "VII" (white dwarf).

<subtype>(number between 0.00 and 9.99)</subtype>

Numberic sub-type of the spectral class. Ignored is spectralType
is set. Values can be given with any precision, but they are
rounded down to the nearest two digits after the decimal dot.

<luminosity>(string)</luminosity>

A valid luminosity identifier. One of the following values:

0, Ia0, Ia-0, Ia+, I, Ia, Iab, Ib, I-II, I/II, Ib-II, Ib/II,
II, II-III, II/III, III, III-IV, III/IV, IV, IV-V, IV/V, V,
VI, VI+, VII

The luminosity type "VI" are subdwarfs, "VI+" extreme
subdwarfs and "VII" white dwarfs.

Ignored if spectralType is set.

<nadirCharge>(true or false)</nadirCharge>
<zenithCharge>(true or false)</zenithCharge>

If there is a recharging station at the nadir or zenith jump point,
respectively. Both default to "false".

<orbitRadius>(number)</orbitRadius>

Orbital "radius" (technically semimajor axis) of the planet's
orbit, in AU. Defaults to roughly the middle of the habitable zone
of the star.

<orbitEccentricity>(number at or above 0.0 and below 1.0)</orbitEccentricity>

Orbital eccentricity, defaults to "unknown", which is assumed
to be at or near 0.0 for all calculations requiring it.

<orbitInclination>(number between 0.0 and 90.0)</orbitInclination>

Orbital inclination in relation to the system's invariable plane.
Defaults to "unknown".

<satellites>(integer)</satellites>

Amount of natural satellites of the planet. Defaults to "unknown"
if no satellite name is set, else the amount of satellite entries.

<satellite>(string)</satellite>

Name of a natural satellite of the planet. Can be given more
than one time if there are more.

<mass>(number)</mass>

Mass of the planet, in Earth masses. Defaults to "unknown"
for display purposes and 1.0 for calculations.

<radius>(number)</radius>

Radius of the planet, in Earth masses. Defaults to "unknown" for
display purposes and 1.0 for calculations.

<density>(number)</density>

Density of the planet in kg/m³. Defaults to "unknown".
Earth has a density of 5515.

<gravity>(number)</gravity>

Surface gravity of the planet, in Earth's gravity. Defaults
to "unknown" for display purposes and 1.0 for calculations.

<dayLength>(number)</dayLength>

Length of day (that is not the same as rotational period!)
on the planet's equator, in hours.

<tilt>(number between 0.0 and 90.0)</tilt>

Axial tilt of the planet, in degrees. Defaults to "unknown"
for display and 0.0 for calculations. Earth's is 23.44.

<className>(string)</className>

Planetary class. Currently unused.

<percentWater>(integer between 0 and 100)</percentWater>

Amount of the surface area covered by liquid water.

<volcanism>(integer between 0 and 6)</volcanism>

Amount of volcanic activity on the planet. Defaults to "unknown".
Earth's is 2.

Planets with volcanism score of 6 are either just forming or in
the process of breaking apart.

<tectonics>(number between 0 and 6)</tectonics>

Relative tectonic activity of the planet. Defaults to "unknown".
Earth's is 2.

Planets with tectonics score of 0 have an unmoving, single crust (but
can still have shield volcanoes). Planets with a tectonics score of 6
have separate "continental" plates the size of small islands.

<landMass>(string)</landMass>

A continent's of major land mass name. Can be given more than once.

<pressure>(integer between 0 and 5)</pressure>

The atmosphere's general pressure category. Defaults to "unknown".
Valid values correspond to the atmospheric types "vacuum", "trace",
"thin", "standard", "high" and "very high".

<pressureAtm>(number)</pressureAtm>

Air pressure in standard pressure units (101325 Pa); defaults to "unknown".

<atmosphere>(string)</atmosphere>

Short description of the atmosphere

<atmMass>(number)</atmMass>

Atmospheric molar mass compared to Earth's standard 28.9645 kg/mol

<albedo>(number between 0.0 and 1.0)</albedo>

The atmosphere's average albedo (amount of sunlight reflected).
Defaults to "unknown". Earth's is 0.33.

<greenhouse>(number)</greenhouse>

The strength of the planet's greenhouse effect (making it warmer
than it should be given its orbital position and albedo).
Defaults to "unknown". Earth's is (currently) 0.12.

<temperature>(integer above -270)</temperature>

The average surface temperature at the equator in °C.

<climate>(string or integer from 0 to 5)</climate>

The planet's overall climate, from 0 (arctic) to 5 (tropical).
Defaults to "unknown" for display purposes and 3 (warm-temperate)
for calculations. Earth's is 3 (warm-temperate).

Alternatively, can be given as one of the following strings:

ARCTIC, BOREAL, TEMPERATE, WARM, TROPICAL, SUPERTROPICAL, HELL

<lifeForm>(string or integer from 0 to 8)</lifeForm>

Highest (native) life form (still not killed off by the humans). An integer
or string from the following table:

0, NONE
1, MICROBE
2, PLANT
3, FISH
4, AMPH
5, REPTILE
6, BIRD
7, MAMMAL
8, INSECT

<habitability>(integer)</habitability>

The base habitability of the planet. This corresponds roughly to how much
human population the planet could support even without modern technology
(see the "pop" tag for details). Defaults to "unknown" and can be negative.

Earth's is typically 7.

<pop>(integer from -1 to 12)</pop>

The planet's population rating. Defaults to "unknown"; -1 means "no population".
The other values describe orders of magnitude:

0: a few people
1: tens of people
2: hundreds of people
3: thousands
4: tens of thousands
5: hundreds of thousands
6: millions
7: tens of millions
8: hundreds of millions
9: billions
10: tens of billions
11: hundreds of billions
12: trillions

Values above 12 make little sense and are displayed as "uncountable" or variant
thereof. Earth's pop value is typically 9 or 10.

<government>(string)</government>

Textual description of the government type. Defaults to "none".

<controlRating>(integer from -1 to 7)</controlRating>

The amount of actual control the nominal government wields over the population.
Tends to drop sharply towards the periphery, with the exception of the clan
homeworlds. Defaults to "situation unclear".

The values correspond to textual descriptions as follows:

-1: total anarchy, not even the government bothers pretending otherwise
0: anarchy, no governmental control outside of the line of sight of its trooops
1: very free society, wild west mentality, everyone is armed, ...
2: free society
3: moderately free society
4: controlled society, strong weapon control laws and surveillance
5: repressive society, secret police, gulags, ...
6: population under total control and surveillance
7: population consists almost entirely of slaves to the ruling class

<socioIndustrial>(string)</socioIndustrial>

The socio-industrial index as defined in the BT publications.
Defaults to X-X-X-X-X.

<hpg>(character)</hpg>

HPG service rating, as defined in the BT publications. Default "X".

<faction>(comma separated list of faction IDs)<faction>

The current factions on the planet. The valid factions are defined in the
data/universe/factions.xml file. Defaults to "no faction".

<desc>(string)</desc>

Free-form description of the planet and star system.

<event>
	<date>(date)</date>
	<message>(string)</message>
	...
</event>

A generic event changing some of the planet's data on the given date. The
date line is required. The message line is optional and makes the event show
up in the daily log, as a news item.

The following data can be changed:

* name
* shortName
* nadirCharge and zenithCharge
* percentWater
* pressure
* pressureAtm
* atmosphere
* atmMass
* albedo
* greenhouse
* temperature
* climate
* lifeForm
* habitability
* pop
* government
* controlRating
* socioIndustrial
* hpg
* faction

<factionChange>
	<date>(date)</date>
	<faction>(comma separated list of faction IDs)</faction>
</factionChange>

Obsolete form of the event tag, only applicable to faction changes. It's
retained for backwards compatibility only and should not be used in
newer data sets.

<delete>(true or false)</delete>

A marker telling the planet data loader to delete the planet with the
given ID from its database, if found. All the other data defined in the entry
is ignored, save for name and id, which are used to identify the entry
to delete. Defaults to "false" (obviously).