All of these are links describing the available species you are able to be.
HumanSangheiliJiralhanae And now for the settings.
Sanghelios
Sanghelios has a gravity of 1.375 Gs, and
an average surface temperature of -5C to
56C (23F to 132.8F). Sanghelios is inhabited
by approximately 8.132 billion Sangheili.
The Bestiarum indicates that Sanghelios is
similar to Earth in shape, and in proportion,
though the land masses are more scattered
an relation to bodies of water. Like Earth, its
atmosphere consists of oxygen, carbon dioxide,
and nitrogen, but also has a reddish hue to it,
perhaps indicating large amounts of iron or
new rock being made by the core.
Being a homeworld, the planet is home to a
huge number of civilian Sangheili numbering over
eight billion, as well as a large defense force.
DoisacFrom the limited details available, it can be estimated that Doisac's environment is very Earth-like, with forests and oceans. It has been noted, however, that its terrain is dominated by magma and magnetism, hinting at high tectonic activity and rich magnetized iron deposits likely caused by the lunar tidal effects projected by its three moons. It is possible that Doisac is closer to Oth Sonin than Earth is to Sol because its surface temperature/climate is slightly more tropical, ranging from -15 °C to 52 °C (5°F to 125.6°F). Alternatively, its atmosphere may cause a natural greenhouse effect, trapping heat and warming up the planet in a similar fashion to Venus.
Earth
Earth, known to prehistoric humans
and Forerunners as "Erde-Tyrene",
is the third planet of the Sol System,
and one of four terrestrial worlds.
Homeworld of the human species,
Earth has been at the forefront of
galactic history, from encounters
with the Precursors to the end of the
Covenant Empire. Inhabited long ago
by a prehistoric interstellar human
government conquered and dissolved
by the Forerunners, Earth is currently
the capital world of the Unified Earth
Government.
Earth was possibly the most populated
planet of the human space territories.
For a significant amount of humanity's
history, it was their only known place in
the universe to support life.
Other available settings:
Reach
Other unnamed planets
Spaaace!