New estimates for Earth-like planets

Researchers have discovered over 300 extra-solar planets (exoplanets). These observations are mainly indirect: one observes the tug of a planet’s gravity on its parent star. Since gravity becomes stronger when separations between massive objects are small, and it is stronger for bigger masses, this “Doppler shift” method is biased towards detecting more massive planets and planets that are closer to their parent stars. Another method that nets us fewer planets but gives us more precise knowledge involves seeing the light from the parent star get slightly dimmer when the planet passes in front of it.

Of course, it would be best to directly see the planets themselves. So far, there is potentially one direct observation of a planet. Potential because these observations usually come with a host of uncertainties.

Regardless of the methods used, astronomers have been astounded at the number of planets that appear to exist and at their variety. We’ve managed to observe planets ranging from a couple times the mass of Earth (which is very exciting!) to several times the mass of Jupiter. The holy grail is to observe a planet of roughly Earth-like mass (could be a few times more massive) in an orbit that is similar to Earth’s around a star similar to our sun. This would be a phenomenal observation because such a planet could easily carry liquid water and all the same elements for life that we have here.

My student Cem, and a former student, Tim both pointed me to an article discussing recent attempts by Astronomers to take all this recent data estimate how many Earth-like planets may be out there:

Frontiers students are often asked to do so-called “back-of-the-envelope” calculations for a variety of things (quick: estimate the amount of carbon emitted yearly!). One fun one that you could try for yourself is to estimate the number of Earth-like planets in the observable universe, given that there are about 100 billion galaxies and about 100 billion stars per galaxy. Enjoy!


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