As a teenager I was fascinated by astronomy and cosmology and read voraciously as well as spending many hours outside at night with my telescope and Norton’s Star Atlas. During my brief years as a physicist in the 1970s, I kept up with the literature and was aware that some astronomers were attempting to detect planets around other stars by detecting the gravitational jiggling of the star around which the planet was orbiting. But these movements were slightly beyond detection by the technology of the day.
Headlines with messages like “First Planet Found Outside Our Solar System” appeared in newspapers dozens of times, at least twice in the New York Times, and once on the front page. But all these announcements were subsequently found to be wrong. And in one notorious incident, it was later found that the astronomer had detected not movements in stars, but movements in the telescope itself. So I was stunned to discover on reading the book “The Stardust Revolution” that new detection methods combined with space-based telescopes had resulted in the proven discovery of nearly 2000 exoplanets.
And not only that, but to identify their approximate sizes, distances from their stars, and orbital speed. And the “solar system” model was not at all standard. There were Jupiter sized planets that orbited their star in less than 5 days, orphan planets in interstellar space, and many Earth-sized planets around Sun-like stars. I realized that I must have been seeing headlines announcing these discoveries, subconsciously thinking “another false announcement” and ignoring them.
Nearly 2,000 planets have been confirmed to be orbiting other stars in our galaxy. NASA’s Kepler mission announced in February this year the discovery of 715 new planets. These newly-verified worlds orbit 305 stars, revealing multiple-planet systems much like our own solar system. Nearly 95 percent of these planets are smaller than Neptune, which is almost four times the size of Earth. This discovery marks a significant increase in the number of known small-sized planets more akin to Earth than previously identified exoplanets.
Four of these new planets are less than 2.5 times the size of Earth and orbit in their sun’s habitable zone, defined as the range of distance from a star where the surface temperature of an orbiting planet may be suitable for life-giving liquid water.
Not only have exoplanets been found, but it has even been possible to obtain a photographic image of a planet. And the Hubble Space Telescope’s resolution and sensitivity have allowed astronomers to discover dusty disks around young stars. These protoplanetary disks are believed to be a circumstellar disk of matter, including gas and dust, from which planets may eventually form or be in the process of forming.
A recent paper1 estimated that there are some 100 million other places in the Milky Way galaxy that could support life above the microbial level, based on a new computation method to examine data from planets orbiting other stars in the universe. The scientists surveyed more than 1,000 planets and used a formula that considers planet density, temperature, substrate (liquid, solid or gas), chemistry, distance from its central star and age. From this information, they developed and computed the Biological Complexity Index (BCI).
The BCI calculation revealed that 1 to 2 percent of the planets showed a BCI rating higher than Europa, a moon of Jupiter thought to have a subsurface global ocean that may harbour forms of life. With about 10 billion stars in the Milky Way galaxy, the BCI yields 100 million plausible planets.
So now we are rethinking our uniqueness. Whereas a few decades back, it seemed entirely possible we may live on the only habitable planet where life has evolved, now it seems more and more likely that habitable planets are very common, and the chemical building blocks of life are generated in huge amounts by stars and in interstellar space. It may well be that the emergence of self-sustaining replicative chemical systems capable of Darwinian evolution is a common phenomenon throughout the universe.
1 Louis N. Irwin et al., Assessing the Possibility of Biological Complexity on Other Worlds, with an Estimate of the Occurrence of Complex Life in the Milky Way Galaxy, Challenges, 2014, DOI: 10.3390/challe5010159 (open access)