Planet-hunters find new worlds in old data

Two scientists have ‘shone a light’ in the foggy spectra of an exoplanetary star system, GJ 676A, potentially revealing two new planets and hints of a third.

An artist’s impression of an exoplanet system. Credit JPL/NASA

Not only did the astronomers confirm the existence of known planet GJ 676Ab, discovered late last year (and five times the mass of Jupiter), the new candidates are only a few times larger than Earth.

What’s more, they made their discoveries by simply tweaking the maths, meaning no new observations or equipment were required. The technique could offer a bumper harvest of new planets for minimal cost and effort.

The spectra came from HARPS (High Accuracy Radial velocity Planet Searcher), an instrument installed at the European Southern Observatory’s 3.6-metre telescope at La Silla, Chile. HARPS detects planets via the radial velocity, or Doppler, method.

In other words it looks for ‘wobbles’ in spectra as stars are tugged gently by orbiting planets over a period of several years. This is more pronounced for smaller, K and M-dwarf stars. HARPS can detect radial velocities down below 1ms-1 making it the most sensitive such instrument in the world.

However, even that seemingly isn’t enough to see everything that’s there. The HARPS method for detecting planets involves comparing sets of spectra separated by time – called cross-correlation.

As straightforward as this is, it doesn’t make the best use of the data. So, Guillem Anglada-Escudé (University of Göttingen, Germany) and Mikko Tuomi (University of Hertfordshire, UK and University of Turku, Finland) developed a more nuanced statistical method; one that seems to shine when it comes to the smaller amplitude signals (i.e. smaller planets) that cross-correlation might miss. It is based on work done by Anglada-Escudé and R. Paul Butler of the Carnegie Institute of Washington called the HARPS-TERRA Software Project.

The first of the new planets found, GJ 676Ae is around 11 Earth masses, with a year lasting 35.5days. The second, GJ 676Ad, is at least 4.5 Earth masses in size and orbits much closer to the star, with a period of 3.6 days. Anglada-Escudé and Tuomi also found a third signal (GJ 676Ac), but as of yet, this is inconclusive.

Crucially though, the astronomers were able to test the robustness of their approach and found it to be strong, statistically. The paper (arXiv:1206.7118v1) suggests directly imaging the larger candidates. If the planets are indeed spotted it would be the ultimate test of the HARPS-TERRA approach; thus proving that there are many more planets out there waiting to be discovered.

Guillem Anglada-Escudé is supported by the German Federal Ministry of Education and Research under 05A11MG3. Mikko Tuomi is supported by RoPACS (Rocky Planets Around Cools Stars), a Marie Curie Initial Training Network funded by the European Commission’s Seventh Framework Programme.

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