Tens of Billions of Earth-like Planets in Milky Way

When the first  extrasolar  planet was discovered about 20 years ago, convention was that the number of these could not be high. But as news after news of fresh discoveries began to flood in parallel to the advances in observation instruments and techniques, we learned to be more modest in our thinking about our place in the universe and our  “privilege”.

Although the number of  discovered planets has now approached 1000, the fact that  most of them were gas giants of  Jupiter size or larger and were either  sizzling  on  orbits almost touching their host stars, or  freezing  on ones  way out, the general feeling until recently was  that Earth-like” planets  able to support  life should still be rare.

Now, however, data from the Kepler Space Telescope have turned that dogma on its head. Astronomers  now believe that not only planets with masses similar to that of Earth or  a few times bigger  make up the  majority of the planets, but  many of them occupy orbits at right distances  from their  stars where life can sprout and flourish.

 

 

 

The team, formed of  the  famed “planet hunter”,   Geoffrey Marcy  of  the California  University (Berkeley) , Erik Petigura, a post-graduate student at the same university and Andrew Howard  from the University of Hawaii  made use of computer algorithms developed by Petigura  for their calculations. The stars the team chose from the Kepler data were “orange dwarf”  stars of spectral class “K”, with masses and surface temperatures slightly lower than those of the Sun.  Petigura also added  “fake” stars to his simulations before running them to see how many could be “nabbed”.

 The researchers  also assumed that  very few of the planets would be at Kepler’s line of sight as they  transit the face of their stars, and when  they added the G-class “yellow” stars like the sun, it was seen that 22 percent of all Sun-like stars would have planets with masses equal  or close to Earth’s  at their habitable zones.

 

The  calculation does not include the relatively smaller and cooler  “red dwarf” stars of spectral class “M”, which make up three-quarters  of all the stars in the Milky Way. But observations  of recent years  have shown that most of the planets with Earth-like masses would be orbiting red dwarfs. For Marcy and his team , the criterion for a planet’s habitability (leaving aside other criteria) is an input of light from the host star which should be between four times and a quarterof what Earth receives from the Sun.

Red dwarfs are deemed problematic  for life because of  their powerful  flares  which may  sterilise the putative planets around them.  Another adverse factor is the proximity of the habitable zones around them, which makes a planet orbiting within it susceptible to be caught in the star’s “gravitational trap”,  with the same side always facing the star and the other being constantly in the dark . Still, as most of the Earth-like rocky planets are thought to be concentrated  around these, they have recently come under the focus of  planetary research and  astrobiology studies. Astronomers  from the Harvard-Smithsonian Astrophysics Center have calculated that  habitable zones around 6 percent of the red dwarf stars could harbour Earth-mass planets. Hence when the numbers of red dwarf stars are considered, the predictions of tens of billions of habitable planets  no longer appear exaggerated. 

Still, Marcy cautions that a planet’s location inside the habitable zone does not automatically make it amenable to life. According to the renowned planet hunter, even if a planet’s  mass and location mimic  those of the Earth, a decisive factor would be its atmosphere, which, if too thick, could make the surface too hot for life, as is the case in Venus, which in other respects, could be viewed as a twin of the Earth.