Earth is the cradle of humankind, but one can’t stay in a cradle forever. The famous words of Konstantin Tsiolkovsky, a Russian rocket scientist born in the 19th century, were perhaps once seen as the mere utterings of science fiction. Tsiolkovsky was a visionary, pointing to space exploration and hu­manity’s eventual expansion at the turn of the 20th century. Since the early 1900s, we have indeed come a long way in realising the Russian scientist’s words.

Since the 1960s we have ventured, mostly via unmanned missions, to several locations in our solar system. From the hot crushing surface of Venus, to the cold Cronian moon Titan, we have landed probes at a number of outposts in the system, with an even larger number of flybys carried out for several other targets. We have even landed members of our own species on the moon.

Several very interesting locations have been uncovered, such as the subsurface oceans of a number of icy moons (the most known being Europa and Enceladus), Mars, with all its outstanding surface features, and Titan, with its glorious landscapes so reminiscent of our own planet. Yet, none of these can be called Earth 2.0, at least not in their current state. Mars’s dry plains, Venus’s intense heat and surface pressure, and the outer planets’ moons’ freezing conditions have ensured that that particular scenario is non-existent.

So where can we look for another Earth? The answer: beyond our solar system. Other stars, just like our sun, may have planets orbiting them as well. Such planets orbiting stars other than the sun are collectively called exoplanets.

In recent years, the observation of exoplanets has become an easier endeavour, significantly because of the transit method. This method involves the indirect observation of exoplanets by viewing and measuring the light received from the star that the exoplanet is orbiting.

When the exoplanet passes bet­ween the star and us here on Earth, we observe the star to dim in bright­ness ever so slightly. By accurately measuring that dip in brightness, we can learn a lot about the exoplanet’s properties. Knowing properties of the star itself, we can determine whether the exoplanet observed is potentially an Earth-like planet or otherwise.

And we have indeed found potentially Earth-like planets! In just a few years, about 4,000 exoplanets in around 3,000 systems have been discovered. The closest of these, Proxima Centauri b, orbits the closest star to the sun, Proxima Centauri, 4.2 light years away.

About 20 per cent of these exoplanets are estimated to be similar in size to Earth and lying in the habi­table zone of their parent star – the distance at which a planet has to be from its star for water to be a liquid on the planet’s surface. If there are around 400 billion stars in the Milky Way, the amount of potentially Earth-like planets could number up to 20 billion – in just our galaxy alone – one of billions of galaxies in the observable universe!

Although the huge distances between stars makes reaching these systems a massive undertaking, we have now come closer than ever to finding Earth-like planets which our descendants might one day venture to.

In the meantime, our searching techniques keep improving and the search continues.

Josef Borg is currently a PhD student at the University of Malta’s Institute of Space Sciences and Astronomy, and also president of the Astronomical Society of Malta.

Sound bites

• Tilted exoplanets could explain odd orbits

Two scientists have suggested that a lot of exoplanets out there are permanently toppled over. Sarah Millholland and Gregory Laughlin (both at Yale University) propose the scenario on March 4 in Nature Astronomy. The idea, weird as it sounds, would solve a longstanding mystery discovered by NASA’s exoplanet-hunting Kepler satellite.

• Mars Insight’s ‘mole’ hits a snag

Drilling operations for NASA’s Mars Insight Lander have been put on hold as the ‘mole’ has hit unexpected obstacles.

Drilling is never easy – especially when you’re trying to do it 172 million miles away on another planet. NASA recently announced that its Mars Insight lander will pause drilling operations for about two weeks, following an unexpected stoppage. Engineers think the halt is due to small stones blocking the drill.

For more soundbites listen to Radio Mocha: Mondays at 7pm on Radju Malta and Thursdays at 4pm on Radju Malta 2

Did you know?

• Rogue planets, orbiting no star, have also been discovered. Such planets are likely to have orbited some star in the past, but as a result of gravitational interactions of their parent star’s activity, these pla­nets were somehow flung out of their system. Such planets can number in the billions in our galaxy as well!

• Extragalactic exoplanets might have also been observed. Using a technique known as gravitational microlensing, astronomers detected a potential exoplanet candidate in the Andromeda galaxy – 2.3 million light years away – with a mass of around six Jupiters! Additionally, planets around stars in the Milky Way which likely origi­nated from other galaxies have also been observed.

• The first reported exoplanet, discovered in 1988, was only confirmed more recently in 2012. In the meantime, several other exoplanets were, of course, detected and confirmed successfully. The first confirmed exoplanet came in 1992, with several terrestrial planets confirmed to be orbiting a pulsar.

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