Star Trek fans may remember that whenever the USS Enterprise orbited an Earth-like world, they announced it as a class-M planet. In the real universe, scientists have what is sometimes called the “Goldilocks Zone” around a star. This is the distance from the star where a planet could sustain liquid water which is essential for life as we know it. In our Solar System, the Earth is accompanied by Mars and Venus inside this zone.
However, as you can probably tell, just being in this zone is not sufficient to guarantee the presence of water. In Venus’s case, an atmosphere rich in carbon dioxide and sulphur dioxide created a runaway greenhouse effect. Radiation coming from the Sun gets trapped inside Venus’ atmosphere heating it up till any water on Venus would boil away.
Mars on the other hand is at the outer edge of the habitable zone and has too thin an atmosphere to capture enough solar radiation. The water on Mars is locked in ice caps.
Spitzer Space Telescope
All this brings us to today’s big news. NASA’s Spitzer space telescope has confirmed the discovery of 7 planets inside the habitable zone around just 1 star, TRAPPIST-1 which lies about 39 light years from Earth. TRAPPIST-1 is an ultra-cool red dwarf star which releases far less energy than our Sun. Consequently, all 7 of these planets are much closer to it than Mercury is to our Sun.
Chart showing the relative sizes of the planets around TRAPPIST-1
The planets themselves are also very close to one another so that from the surface of one world, some of the neighbors could appear as large as the Moon appears to us.
To put this discovery into context, till the 1980s, astronomers hadn’t reliably discovered any planets outside our own solar system. The farthest planet in our solar system, Neptune is 4.7 billion km away from us. This is less than 0.0005 light years. In contrast, the nearest star outside the solar system, Proxima Centauri is 4.243 light years away.
I wanted to write a post about how scientists search for exoplanets. But it turns out NASA’s Jet Propulsion lab has a very good site here. which explains this with very helpful illustrations. I’ll just settle for giving you a summary. There are many ways to detect an exoplanet:
- From the star’s wobble: Gravity causes a planet to revolve around a star. The same gravitational pull also acts on the star causing it to wobble (although with a much smaller radius than the smaller planet). Wobble caused by large planets are detected in two ways:
- Astrometry: Taking pictures of the star repeatedly and seeing if it moves relative to fixed stars in its neighbourhood.
- Doppler shift: While wobbling, when the star moves away from us, the light coming from the star gets slightly redder and when it is moving towards us its light gets slightly more violet due to s phenomenon called the red shift. This can be accurately measured to tell about the planets around it.
- From the star’s eclipse: If the planet’s orbit takes it in between the star and us, it will dim the light coming from the star the same way as the moon stops sunlight during an eclipse. This is by far the most common way of finding exoplanets and more planets have been found this way than all other methods combined.
- The planet’s Gravitational lensing: Einstein’s theory of Relativity predicts how the light from a distant star can get bent from its straight path by an intervening star or planet. Scientists study the bending caused by a nearby star on light coming from a much farther star. If the nearby star also has a planet around it, its effect on the light may also show up.
- Directly looking at it: The light reflected from a planet is usually swamped out by the much brighter light coming from its star. But if we block the light coming from the star (the same way as we use a sunshade to block the sun when looking at something on earth), the planet may become visible.
So far, there 3,449 planet discoveries outside our solar systems have been confirmed. The latest discovery of 7 are special however as they are around a red dwarf star. Almost 75% of the stars in our galaxy are red dwarfs and they last longer too. This opens the possibility that rocky Earth-like planets may be far more common than previously thought.