Billions of planets with life — or just one?

A comparison of Earth, Neptune and exoplanet Phobetor. Image by Aldaron

Around a star called Lich, in the constellation of Virgo the Virgin, orbit three planets called Poltergeist, Phobetor and tiny Draugr – really, I am not making this up! Except that Lich is no longer a star, because it exploded, briefly becoming brighter than a thousand suns, blasting any planets’ atmospheres into space and certainly frying any life they may have had.

What was left of the star collapsed down to a ball about 20 kilometers in diameter, more massive than the sun — a pinhead would be a million tons! — and spinning at 10,000 rpm, seen on Earth as radio pulses: a pulsar.

The spin rate was more precise than an atomic clock, but close inspection showed tiny periodic variations that could only be explained by the gravitational pull of planets. Lich’s planets the first to be discovered outside our solar system, called extrasolar planets, or exoplanets.

Like sunlight spread in a rainbow, starlight, when analyzed in a precise spectrograph, shows lines like a barcode, which shift toward blue if the star approaches and toward red if it recedes. In 1995 a star not very different from the sun was found to wobble back and forth as it was pulled by a big planet, a “hot Jupiter,” orbiting every 4.23 days.

The technique improved so much that astronomers can now detect walking-speed wobbles, and hundreds of exoplanets have been found this way, including some similar to Earth in size. But even more have been found in transit, that is, by passing in front of the star and dimming its light. Venus and Mercury sometimes do that to our sun, but we can see those as little black discs.

As of Sept. 8 we have found 3,667 exoplanets, with 616 stars having more than one, mostly using the Kepler telescope in space, monitoring 145,000 stars for periodic dimming.

Most exciting so far is Trappist 1, a star with seven planets found, at least three of which are in the Goldilocks zone: not too hot or cold but just right for liquid water, and perhaps life.

Sometimes we can even detect a planet’s atmosphere as the starlight shines through it.

On Aug. 31 astronomers using the Hubble Space Telescope reported data suggesting large quantities of water on those three planets.

It now seems that there may be more than a billion Goldilocks planets in our Milky Way galaxy. We know one has life. Is it just one, or millions, or billions?