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4 mins read 08 Jun 2020

Ruling Out Fake Exoplanets

A team of astrophysicists have demonstrated that out of 224 potential exoplanet candidates, that 30 of them are likely to actually be stars.

Different views of a transiting exoplanet, reflecting the light curve dip of starlight - measured by telescopes. Credit: Viewspace.org

Our night sky is not only filled with multitudes of stars but also exoplanets. Trying to identify these other worlds, all orbiting distant suns outside our Solar System, can be difficult - as scientists mostly observe them indirectly through their influence on their host star. These techniques often involve measuring the ‘wobble’ in the star’s orbit, caused by the gravitational influence of the planet - and in some cases, the minute dip in starlight received by telescopes, as planets transit across their surface from our vantage point of view here on Earth. 

In a new paper, a group of astrophysicists, including author Jake T. Clark from the University of Southern Queensland (USQ), have revised 224 potential exoplanets to confirm their status. 

The paper, titled “K2-HERMES II. Planet-candidate properties from K2 Campaigns 1-13”, consolidates data from multiple sources in order to determine whether the identified targets were in fact exoplanets.

Clark states that he was excited for other astronomers to utilise this knowledge. “We've worked pretty hard to obtain this new and exciting information on these planets and potential planet-hosting stars, and I'm really keen to see how it's used within the exoplanet community,” he said.

How to find exoplanets

Exoplanets can be indirectly observed using the transit technique. When a planet orbits a star, assuming that we are in the correct line of sight, we can see the planet pass in front of the star, similar to eclipses we see on Earth. When the planet passes, or transits, in front of the star, the light from the star dims slightly. This property can be measured, and so we can tell if a star has an orbiting planet, and also the size of the planet relative to the star. The Kepler-2 (K2) mission identified more than 1000 confirmed and candidate (unconfirmed) exoplanets by using this transit technique.

Information was also gathered using Australia’s largest optical telescope, the AAT (Anglo-Australian Telescope), and HERMES (High Efficiency and Resolution Multi-Element Spectrograph). This data helped to calculate the radii of the targets, and therefore determine candidates that were realistically exoplanets. In total, the K2-HERMES program observed a total of 224 planet candidates orbiting 119 stars, all of which were used in this study. These stars are of various types but they are all on the main sequence, meaning that they fuse hydrogen into helium as their source of fuel. 

The Anglo-Australian Telescope at Siding Spring Observatory. Credit: Australian Astronomical Observatory.

Putting it all together

Upon consolidating information from the K2 mission and AAT using HERMES, the team was able to determine that out of 224 planet candidates, 30 of these were likely to actually be stars as their radii were greater than two times that of Jupiter. 

Research such as this expands our capabilities of finding exoplanets, including those that are Earth-like, however, we still don’t know much about them. Clark emphasises, “There's just so much we're still unsure about! If we truly want to start finding planets that are analogous to our own, then there's still a lot of work needed. From instrumentation to scientific models and observations, we're still a good five to ten years away from finding planets that would resemble our own”.

Australia is playing a strong role in the discovery of exoplanets. “In the last two years, we've commissioned an instrument upon Australia's largest telescope, the AAT called Veloce, and started up an entire observatory in regional Queensland, Minerva Australis, both dedicated to finding and characterising exoplanets for NASA's new planet-finding mission, TESS,” said Clark. “TESS has already discovered thousands of potential planets, which are already being followed up with Minerva-Australis and Veloce.”

The results of Clark and his team’s research emphasises the importance of observing stars in order to determine the properties of the planets that orbit them, and demonstrates Australia’s role in finding and researching exoplanets. It also goes to show that even though we can only indirectly observe exoplanets, it does not mean that we cannot explore them.  

The paper is currently available on arXiv.org