Australasian Science: Australia's authority on science since 1938

Mysterious Signals from Beyond Our Galaxy

By David Reneke

Mysterious signals have been detected from beyond our galaxy, and an exoplanet’s size has been measured to an accuracy of 1%.

A split-second burst of radio waves detected by the Arecibo radio telescope in Puerto Rico provides important new evidence of mysterious pulses that appear to come from deep space. They mark the first time that a so-called “fast radio burst” has been detected in the Northern Hemisphere of the sky.

Fast radio bursts consist of bright flashes of radio waves lasting only a few thousandths of a second. Scientists using the Parkes Observatory in Australia have recorded such events for the first time, but the lack of any similar findings by other facilities led to speculation that the Australian instrument might have been picking up signals originating from sources on or near Earth.

“The characteristics of the burst seen by the Arecibo telescope, as well as how often we expect to catch one, are consistent with the characteristics of the previously observed bursts from Parkes,” says Dr Laura Spitler of the Max Planck Institute for Radio Astronomy.

The results eliminate any doubt that these radio bursts have come from far outside our galaxy, but it’s too early for that sort of speculation. Exactly what may be causing them represents a new enigma for astrophysicists. Possibilities include a range of exotic astrophysical objects, such as evaporating black holes, mergers of neutron stars or flares from magnetars – a type of neutron star with extremely powerful magnetic fields.

These strange cosmic bursts occur roughly 10,000 times per day over the whole sky and appear to be coming from beyond the Milky Way galaxy, from the direction of the constellation Auriga. Future instruments, such as the Square Kilometre Array and its pathfinders, promise to be efficient fast radio burst detectors that will vastly expand the knowledge of this phenomenon.

Sizing an Alien World

A team led by Sarah Ballard of The University of Washington has measured the diameter of a “super Earth” about 300 light years away to within an astounding accuracy of 235 km, or about 1%. To size up the planet, named Kepler-93b, Ballard used data from NASA’s Kepler and Spitzer Space Telescopes to determine a diameter of 198,800 km.

Kepler discovered the planet, which passes directly in front of its parent star, causing the starlight to dim during the transit. That dimming, which occurs once per orbit, is what allowed Kepler mission scientists to find the planet in the first place.

Both Spitzer and Kepler then recorded multiple transits at visible and infrared wavelengths. Data from the observatories confirmed that Kepler-93b really was a planet and not some artefact of stellar variability.

Ballard then knew that by looking carefully at the light curve she could calculate the size of the planet relative to the star. The only missing piece was the diameter of the star itself. This was measured using a breakthrough technique called asteroseismology.

Most people are aware of seismology – the study of seismic waves moving through the Earth. Asteroseismology is essentially the same thing, except it’s used for stars.

The outer layers of stars boil like water on top of a hot stove. The convective motions create seismic waves that bounce around inside the core, causing the star to ring like an enormous bell. Kepler can detect that ringing, which reveals itself as fluctuations in a star’s brightness.

Ballard’s colleague, Prof Bill Chaplin of The University of Birmingham, led the asteroseismic analysis for Kepler-93b. By analysing the seismic modes of the star, he was able to deduce its radius and mass to an amazing accuracy of a single per cent.

The new measurements confirm that Kepler-93b is a “super-Earth”-sized exoplanet, with a diameter about one-and-a-half times the size of our planet. Previous measurements by the Keck Observatory in Hawaii had put Kepler-93b’s mass at about 3.8 times that of Earth. The density measurements suggest that the planet is very likely made of iron and rock, like Earth itself.

Although super-Earths are common in the galaxy, none exist in our solar system. That makes them tricky to study. Ballard’s team has shown, however, that it is possible to learn a lot about an exoplanet even when it is very far away.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com