In February 2011 the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 54 that may be in the habitable zone. Six of the candidates in this zone are smaller than twice the size of Earth. A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported. Based on the findings, the Kepler Team estimated there to be “at least 50 billion planets in the Milky Way” of which “at least 500 million” are in the habitable zone.
Small planets mean geologically dead & reduced tectonic activity
Radical axial tilt makes seasons extreme and make bio-homeostasis more difficult
Red dwarfs emit little heat – to be close enough for life, a planet could well get tidal (gravitational) locking. Further, close neighbours might disrupt the stability of various orbiting bodies such as Oort cloud and Kuiper belt objects, which can bring catastrophe if knocked into the inner Solar System. Why? Because of the quantity of large mass objects. A gas giant close to the sun will disrupt the habitable (zone of the earth). However if it were reduced as say Venus…
Water on Earth
Volcanic outgassing could not have accounted for the amount of water in Earth’s oceans. The vast majority of the water – and arguably carbon – necessary for life must have come from the outer Solar System, away from the Sun’s heat, where it could remain solid. Comets impacting with the Earth in the Solar System’s early years would have deposited vast amounts of water, along with the other volatile compounds life requires (including amino acids) onto the early Earth, providing a kick-start to the origin of life. Thus, while there is reason to suspect that the four “life elements” ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.