Exciting find

ASTRONOMY

The reported discovery of an earth-like planet orbiting our nearest stellar system at a ‘spitting distance’, as astronomers like Marcy have termed it, has revived a space flight to Alpha Centauri, writes C Sivaram

In the past 15 years, several hundreds of exoplanets have been detected orbiting near and the not-so-near stars. Some stars like 55 Cancri have many planets orbiting them, like our own solar system. Recently, this star was found to have another planet, about the size of the earth, but searingly hot and suspected to be almost like a diamond in structure.

Many other exotic planets with several times the earth’s water content have also been found. The Kepler satellite, orbiting the earth for the past three years, is expected to show a considerable increase in the number of exoplanets.

Recently, there was an announcement of the discovery of a planet about the earth’s mass, circling our nearest stellar neighbour, Alpha Centauri B, hardly a little more than four light years away. The Alpha Centauri is a triple star system. The main components, Alpha Centauri A, slightly larger and more luminous than the sun, and Alpha Centauri B, which is slightly smaller, are close companions, orbiting each other with a period of 80 years.

They come as close as 15 billion km to each other. In turn, these two stars are being circled at a much greater distance of some 1.5 trillion km by a dwarf star known as Proxima Centauri since it is slightly closer to the earth than the other two, due to this trillion-km difference. This new exoplanet has now been observed circling Alpha Centauri B with a period of hardly three days, implying that it is just six million km from its parent star.

This proximity would result in the planet having hellish temperatures of over a thousand degrees. The planet was detected by the ‘wobble method’, using the HARPS spectrograph at European Southern Observatory, which measures the minute wobble imposed on the host star by the to and fro tug of the planet’s gravity. This wobble, caused by the tug and imparting a minute velocity of 50 cm per second, was measured over nearly 500 observations over four years.

This velocity tug implied a planet mass, just about that of the earth. Unfortunately, this planet is not in the habitable zone of the Alpha Centauri B, where temperatures are cool enough for water to exist as liquid and which is about a 100 million km from the star (implying an orbital period of 200 days for such a planet). For a similar, small tug, a planet in the habitable zone would have to be four times more massive than the earth. In our solar system, earth is the only habitable zone.

So with more precise measurements, it may be possible to locate more planets in the habitable zone, in the stellar system closest to us. Only about 50 exoplanets are known to be in the habitable zone of their host star. James Cameron’s famous movie ‘Avatar’, featured a beautiful exomoon called Pandora, orbiting the planet of Alpha Centauri B. Such exomoons are difficult to detect even at the close distance of Alpha Centauri. The angular size of our moon at that distance would be less than 0.2 micro arc seconds. It is about hundred times smaller than what is possible with current technology.

Voyager spacecraft

The reported discovery of an earth-like planet orbiting our nearest stellar system at a mere ‘spitting distance’, as astronomers like Marcy have termed it, has revived suggestions for a space flight to Alpha Centauri. There have been calls for an international project to launch a scientific space probe to Alpha Centauri. The Voyager spacecraft has traversed hardly 20 billion km over a period of 35 years.
It will need at least a hundred thousand years to cover the distance to the nearest star. Faster trips consume much more energy. For instance, the energy to be supplied per unit mass of payload for a few decades’ trip to the nearest star would be a billion times larger than the energy for the Apollo manned lunar flight.

There are some plans of a powerful laser beam operating from space, propelling a 30-tonne spacecraft with radiation pressure from the laser, providing thrust by impinging on a large sail of 30-km diameter.  If all the power generated by mankind (currently ten terra watts) is continuously used in such powerful laser operation, it can propel a spacecraft to one-fifth light velocity and reach Alpha Centauri in 50 years.Of course it is well beyond current technology. Considering all the power shortage, who would want to expend the entire power output of mankind for such a project. At that speed, the spacecraft has a kinetic energy of 30 million times required to escape earth’s gravity.

Even if one uses thermonuclear energy to propel this spacecraft, if a one-tonne space probe is to reach Alpha Centauri in 50 years, it would consume several thousand tonnes of such fuel and power consumed by the whole of mankind in several hundred years.  The cost would be something like a trillion crore rupees, and just for a one-way trip.

If our spacecraft must stop at its destination, take off again and return to earth, the energy required would be several million years of the earth’s power consumption.  Indeed there has been a proposal of Project Daedalus and its recent avatars, which is a hypothetical ‘worldship’ intended for a trip to Barnard’s Star, six light years away.
The spaceship is conceived to be about half a km long, weighing 50,000 tonnes and propelled by nuclear fusion energy supplied by the isotope Helium-3. The spacecraft will have to harvest this nuclear fuel in Jupiter’s atmosphere where it is supposed to be in abundance.

The ship is to be nuclear propelled for four years to a speed of about 130 million kilometers per hour. The total trip time is estimated to be 50 years. It was suggested that Daedalus be built sometime in the 21st century on the moon. The shorter the trip duration, the more the cost. With current space technology, space probes to even the nearest star would take several thousands of years to reach.