GJ 1151, your secret’s out

THERE are several ways in which astronomers can detect the presence of planets orbiting other stars, and the general subject has come up many times in my columns.

Now, astronomers have used a new method. For the first time, radio astronomy has been used to discover the likely presence of a planet orbing the red dwarf star GJ 1151. Significantly, the planet is likely to be roughly the same mass as Earth.

It’s worth reminding ourselves of the two main methods used so far, because one of them has been used – at least in a negative sense – to make supporting observations that restrict the mass of the newly discovered planet.

The majority of planets orbiting other stars have been found by watching for drops in light exhibited by their ‘parent’ stars as the planets pass in front of them. However, the earliest method – and one which is still very important – was to examine the wobbles in the stars’ motions caused by the gravity of a planet, or planets.

Now, radio astronomy has been used. The observations have revealed radio emission that is very likely to come from the interaction between the star and its planet. The work has been done using LOFAR, the low-frequency radio astronomy array that is spread across a large area in Europe.

The majority of antennas are in the Netherlands; others are located in Germany, Britain, France, Ireland and Sweden.

The general idea is that the observed emission arises because of the star’s strong magnetic field. The star is a red dwarf star: although they are much less massive than our sun, they possess stronger magnetic fields. When a magnetic field, carrying a stream of charged particles, encounters an obstacle such as a planet, an effect called an electron cyclotron maser instability (ECMI) can take place.

The idea that such an interaction could occur is supported by observations that suggest that the planet orbits GJ 1151 over a period somewhere between two and five days. Such a short period tells astronomers that the planet orbits quite close to the star.

The LOFAR antennas are very sensitive. Indeed, it is the only current detector system around the 150 MHz frequency that is sensitive enough to have performed this work.

GJ 1151 is not the only red dwarf star to have been detected at radio wavelengths: it is not uncommon for this type of star to have flares, which have been observed in this way. However, in the case of this star, the radio emissions have lasted about eight hours, which is indicative of a star-planet interaction. In addition, the fact that the waves are circularly polarised is another indication that astronomers have the correct interpretation of the observations. (Circular polarisation means that instead of the electric waves vibrating in only one plane, they rotate as they travel through space.)

Researchers have deduced that the planet is not a very massive one like Jupiter; it is likely to have a mass only up to a few times that of Earth. They know this because independent observations have detected no ‘wobble’ in the star’s motion, which puts an upper limit on the planet’s mass.

On a smaller scale, this is rather like the interactions in our own solar system that take place between Jupiter and its moon Io. Jupiter has a strong magnetic field, and these effects are well known.

Indeed, researchers have been using the Jupiter observations as a model for what is happening with GJ 1151.

Here in Tasmania, significant radio observations of Jupiter were made, beginning in the 1960s, by researchers at the University of Tasmania, working near Hobart Airport.

Some of these related to the Jupiter-Io interactions, and I have no doubt that the results they obtained have formed an important part of the background to the interpretation of these modern observations.

In addition to the emissions that related to the Jupiter-Io interaction, the university researchers also used Jupiter emissions to help pin down Jupiter’s rate of rotation.

The detection of this planet is a first for radio astronomy, opening up a new window in the detection and study of extrasolar planets.