Astronomy is one of our oldest sciences. Humans have studied the stars for all of recorded human history and quite possibly well beyond. After all, stars are a nightly occurrence and hard to ignore.
Our ancestor built a great deal of lore around the stars. In Eastern, Western, and ancient American culture, wise men studied these points of light hoping they might reveal what the future holds.
But to the naked eye, there are five stars which do not behave like any other. They move against the celestial backdrop is a wayward manner. The "wanderers" or planets as they were called held a mystical place in some religions, developing into astrology or the belief that the fate of humans could be read in the stars.
Of course, the invention of the telescope allowed astronomers such as Galileo to show the wanderers were planets with their own orbiting bodies. It is difficult for anyone to look at Jupiter on a clear night, see its moons with their own eyes and then deny the truth of the solar system model he propounded.
The discovery of Uranus, Neptune, Pluto, and the asteroid belt led to a much clearer picture of our local neighbourhood. In the past few decades, probes have flown past every planet and provided us with close up images of these distant worlds. We have even landed probes on Venus and Mars - although the Venusian probes only last a matter of minutes.
The continuing exploration of Mars has revealed much about the solar system. While alien life has not been detected, there is certainly evidence water flowed openly on the surface of Mars and may still be found in underground caches. It is doubtful Mars will ever be a habitable planet for the likes of you and me but life might have found a way.
Of course, exploration of the solar system led to questions. Are we unique? Are there solar systems around other stars? And if so, could they possible have the same conditions we have here on Earth? Is there life out there in the galaxy?
The first detection of a planet outside of our solar system likely took place in 1917 but the astronomers at the time did not recognize it as such. The first confirmation of an exoplanet - a planet orbiting another star - was reported in 1992.
This was followed by the confirmation of other observations made four years earlier. Other stars definitely have planets.
As of this month, a total of 4,109 exoplanets have been confirmed in 3,059 different star systems. A total of 667 systems have been shown to have more than one planet. These numbers will only increase as astronomers refine their approaches. Indeed, by looking at electromagnetic wavelengths outside of the visible region of the spectrum, using transit photometry, and employing Doppler spectroscopy, it is likely we will find planets around most of the 200 billion stars in our galaxy. It has been hypothesized there are 11 billion potentially Earth-like planets in the Milky Way and we inhabit only one of well over 100 billion galaxies in the universe.
The question - for astronomers - is no longer "are there other Earth-like planets?" but rather "where should we look first?"
With this in mind, last week two groups of astronomers announced "water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18b".
Why did they focus on this particular planet? A number of reasons.
It was first discovered by the Kepler space telescope program in 2015, orbiting a red dwarf star (K2-18). The star is 124 light years from Earth and the planet was first detected by variations in the light curve for the star. Subsequent data from the Spitzer Space Telescope confirmed K2-18b orbits in the habitable zone around its star with a 33 day period. Each year on the planet would be a little over a month long - although it is likely tidally locked and therefore always has the same face pointing at its Sun.
The short orbit worked to the advantage of the groups analyzing the planet. Each transit across the face of its star allowed more data to be collected greatly reducing the signal-to-noise ratio and enhancing the ability of the teams to ascertain the components of the atmosphere. The planet's equilibrium temperature is estimated to be -8 plus/minus 5 degrees Celsius. The error bars, the assumptions about the size and mass of the planet, and the fact it is tidally locked all argue for a surface temperature which is in the "goldilocks zone."
Finally, studies with the Hubble Space Telescope corroborate the Kepler and Spitzer results and confirm the presence of water vapour as detected in the spectra of the starlight passing through the planet's atmosphere during a transit.
Our ancestors might have looked to the night sky with wonder but we now know there is more out there than meets the eye. And likely many more discoveries yet to come.
