I see announcements of “Earth-Like” planets announced almost daily.  The problem is that they are not even Earth like at all and they most likely are not habitable.  70% of observable stars are “M” type stars, basically red dwarfs.  It’s easier to find Earth sized planets around red dwarfs by the wobble method because a lower mass star takes less mass to move it a detectable amount.  It’s easier to detect Earth size planets by the transit method if the orbit is in our line of sight because they block a larger percentage of the light than they would on a larger star.

     Our star is a “G” type star, this means it is approximately 4x more massive and 1000 times more luminous than these red dwarf stars.  Earth sized planets are more difficult to find around “G” type stars because the greater mass of the star makes the wobble induced by an Earth size planet smaller and difficult to differentiate from noise.  When detected by the transit method a smaller percentage of the stars light is blocked making it less noticeable.

     The habitable zone of a planet is defined as that distance from it’s parent star where water can exist simultaneously in all three forms, gas, liquid, and solid.  Liquid water seems to be an essential ingredient of life.  Understandably, the habitable zone would be closer to a smaller star.  So many of these “Earth like” planets are orbiting these red dwarfs at distances that might make the year anywhere from around 4 days to 30 days depending on the star.  But Mercury with an orbit of 88 days is not far away from being tidally locked, it is rotating 3x for every 2 orbits.  A planet less than a 30 day orbit from it’s parent star is almost sure to he tidally locked.

     Tidally locked is the limit of the habitable zone.  Even though a suitable temperature range would exist on the terminator, that part of the planet where the sun is half-set, No liquid or solid water would be there because anywhere water could become a vapor, it would be carried to the dark side where it would freeze out and remain forever frozen.  Hence the atmosphere of such a planet would be extremely arid as would the baked land.  So this would limit habitability.

     Another issue with tidally locked stars regarding habitability is that of the planets magnetic field.  Since the planet would be spinning much slower, essentially at one revolution per year, however many days that is, the planet would generate a much weaker magnetic field.  Our planet’s magnetic field protects us from radiation in forms of alpha and beta particles as well as relativistic protons by deflecting these particles to the Earth’s poles.  With a very slow spin the magnetic field may not be strong enough to substantially deflect these forms of radiation.  Also Red Dwarfs are known for being less peaceful and more variable than our star, more violent flares and star-spots, more violent solar radiation.

     It is possible in a red dwarf planet that you could have a much thicker atmosphere that would distribute heat even though the planet was tidally locked, and, if this atmosphere had enough greenhouse gases like carbon dioxide and methane, then that would extend the distance from the red dwarf that is habitable and might extend it beyond the range of tidal locking.

     But for these reasons I don’t get two excited when I hear about the discovery of a “Earth like” planet around a red dwarf star.

     One could speculate about an “Earth like” planet around a “K” type star and this would seem more likely than “M” type.  But I will really get excited when I hear of an Earth like planet around a “G” type star like the Sun, in a similar orbit to Earth.  Well as it happens there are two Super Earth’s around Tau Ceti, a “G” type star  like our Sun.  It is .78% the mass of the Sun and slightly cooler, surface temperature of around 5300k verses 5600k for our Sun.  And it is only 12 light years from Earth.  It may be a viable place for an alien race to originate.