It’s another world out there

THESE days, it’s nothing new to hear of the announcement of a new planet having been discovered. Since the first planet around a normal star was discovered in 1995 – the planet called 51 Pegasi b – the total of known planets orbiting other stars has reached 4301, as of the middle of this week.

However, something new and exciting has now happened. For the first time, astronomers have directly imaged a pair of planets orbiting a star similar to our Sun.

Of all these “extrasolar” planets that we have detected, we have images of relatively few of them. This is because the stars are very distant. Planets orbiting them would appear very close to them, and in general they would be lost in the glare of the star itself.

Perhaps you have been up before the sun recently, admiring the brilliant planet Venus in the north western morning sky.

It’s easy to see, but try to imagine how difficult this would be if it was almost in the same direction as the Sun!

The exciting image of the two planets was obtained using instruments mounted on the Very Large Telescope (the VLT) in northern Chile, in the Atacama Desert.

The date was February 16 this year, and the telescope was aimed at the star TYC 8998-760-1, about 310 light years away. To see the planets, the main glare of the star had to be blocked.

One of the two planets (TYC 8998-760-1 b) was already known, but the image showed a second one, orbiting about twice the distance of the first.

Although I write “orbiting”, astronomers have not actually been watching the planets orbiting their star. They are a very long way from the star, and their orbital motion is too slow for us to currently detect.

Using the Sun-Earth distance in our Solar System as what astronomers call the astronomical unit, the inner one is about 160 astronomical units from its star, and the recently discovered one, called TYC 8998-760-1 c, is about twice this distance out. Indeed, the distances could be even greater, as we may not be seeing the system exactly “face on”.

These distances, and the blocking of the light of the main star, are two factors that helped astronomers make the detection.

The great orbital distances make even the dwarf planet Pluto’s distance from the sun seem small by comparison: Pluto orbits, on average, about 40 astronomical units out.

The farther a planet from its “parent” star, the longer the orbit takes. Pluto’s orbital period is 248 years, but the great orbital distances of these extrasolar planets mean that they would take thousands of years to complete each passage around their star.

So astronomers need to resort to a different technique to determine whether two or more points of light are likely to be travelling together through space.

It is possible, of course, for two objects to appear close together in the sky, when in fact they are at vastly different distances, simply being seen along nearly the same line of sight. To be able to discount this possibility, astronomers measure the change in direction – called proper motion – of the objects as they drift through space. If they appear to be drifting in the same direction and at the same angular speed, this is very strong evidence that they are actually physically associated with each other.

As so often happens in astronomy, an earlier image also showed the system, even though the “c” star was unnoticed at the time. This one was made in 2017, and by comparing the images taken more than two and a half years apart it was possible to see that they were all moving together. This is called common proper motion, or cpm.

There have been many “firsts” in astronomy, and they keep coming. Soon, we expect to be seeing images taken from the first helicopter on Mars!

Martin George is manager of the Launceston Planetarium (QVMAG)