The nearest Earth-like planet

What could be more exciting for exobiologists and planetary scientists than to discover that a nearby star is orbited by a planet approximately the same mass as the Earth that may support liquid water: a world in the ‘Goldilocks zone’? It seems that Proxima Centauri, the Sun’s closest companion star (4.2 light years distant), might have such a planet (Anglada-Escudé, G. And 30 others 2016. A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature, v. 536, p. 437-440).  It is one of 34 candidates found to date with various levels of likelihood for having the potential to produce life and support it. To fit the bill a planet first has to orbit a star at a distance where the stellar energy output is unlikely to vapourise any surface water yet is sufficient to keep it at a temperature above freezing point, i.e. the ‘Goldilocks’ or circumstellar habitable zone is closer to a cool star than to a hot one. Note that the liquid-water criterion requires that the planet also has an atmosphere with sufficient pressure to maintain liquid water. It also needs to have a mass close to that of the Earth (between 0.1 to 5 Earth masses) and a similar density, i.e. a candidate needs to be dominated by silicates so that it has a solid surface rather than being made mainly of gases and liquids.

The location of Alpha Centauri A and B, Proxim...
The location of Alpha Centauri A and B, Proxima Centauri and the Sun in the Hertzsprung-Russell (HR) diagram. (credit: Wikipedia)

Proxima Centauri b, as the planet is called, was not discovered by the Kepler space telescope using the transit method (drops in a star’s brightness as a planet transits across its disk) but by terrestrial telescopes that measure the Doppler shifts in starlight as it wobbles because of the gravitational affect of an orbiting planet. As well as being close, Proxima Centauri is much smaller than the Sun so such effects are more pronounced, especially by planets orbiting close to it. The planet that has excited great interest has an orbital period of only 11.2 Earth days so is much closer to its star and may have a surface temperature (without any greenhouse effect) of 234 K (21 degrees less than that of Earth). The wobble suggests a mass and radius are likely to be 1.3  and between 0.8 to 1.4 times those of Earth, respectively. So Proxima Centauri b is probably a silicate-rich world. But, of course, such limited information gives no guarantee whatever of the presence of liquid water and an atmosphere that can support it. Neither is it possible to suggest a day length. In fact, such a close orbit may have resulted in the planet tidally locked in synchrony with its orbit, in the manner of the Moon showing only one face to the Earth. Moreover, its star is a red dwarf and is known to produce a prodigious X-ray flux, frequent flares and probably a stream of energetic particles, from which only a planet with a magnetic field is shielded. All red dwarfs seem to have such characteristics, and the list of possible Earth-like planets show them to be the most common hosts.

It is too early to get overexcited as technologies for astronomical detection of atmospheres and surface composition are about a decade off at the earliest. Being so close makes it tempting for some space agency to plan sending tiny probes (around 1 gram) using a laser propulsion system that is under development. Anything as substantial as existing planetary probes and certainly a crewed mission is unthinkable with current propulsion systems – a one-way trip of 80 thousand years and stupendous amounts of fuel.

Planet Mercury and giant collisions

Full-color image of from first MESSENGER flyby
Mercury’s sun-lit side from first MESSENGER flyby (credit: Wikipedia)

Mercury is quite different from the other three Terrestrial Planets, having a significantly higher density. So it must have a considerably larger metallic core than the others – estimated to make up about 70% of Mercury’s mass – and therefore has a far thinner silicate mantle. The other large body in the Inner Solar System, our Moon, is the opposite, having the greatest proportion of silicate mantle and a small core.

The presently favoured explanation for the Moon’s anomalous mass distribution is that it resulted from a giant collision between the proto-Earth and a Mars-sized planetary body. Moreover, planetary theorists have been postulating around 20 planetary ‘embryos’ in the most of which accreted to form Venus and Earth, the final terrestrial event being the Moon-forming collision, with smaller Mars and Mercury having been derived from the two remaining such bodies. For Mercury to have such an anomalously large metallic core has invited mega-collision as a possible cause, but with such a high energy that much of its original complement of silicate mantle failed to fall back after the event. Two planetary scientists from the Universities of Arizona, USA, and Berne, Switzerland, have modelled various scenarios for such an origin of the Sun’s closest companion (Asphaug, E. & Reuffer, A. 2014. Mercury and other iron-rich planetary bodies as relics of inefficient accretion. Nature Geoscience, published online, doi: 10.1038/NGEO2189).

Their favoured mechanism is what they term ‘hit-and-run’ collisions in the early Inner Solar System. In the case of Mercury, that may have been with a larger target planet that survived intact while proto-Mercury was blasted apart to lose much of it mantle on re-accretion. To survive eventual accretion into a larger planet the left-overs had to have ended up in an orbit that avoided further collisions. Maybe Mars had the same kind of lucky escape but one that left it with a greater proportion of silicates.

One possible scenario is that proto-Mercury was indeed the body that started the clock of the Earth-Moon system through a giant impact. Yet no-one will be satisfied with a simulation and some statistics. Only detailed geochemistry of returned samples can take us any further. The supposed Martian meteorites seem not to be compatible with such a model; at least one would expect there to have been a considerable stir in planetary-science circles if they were. For Mercury, it will be a long wait for a resolution by geochemists, probably yet to be conceived.