Season 17, Episode 66: How Do We Know There’s One Planet Per Star on Average?

Hey StarTalkians! Season 17, episode 66 was an exoplanet-focused discussion between Neil, co-host Matt and guest Professor Evgenya Shkolnik. Early in the discussion, they discuss a fact that comes up on StarTalk a lot: that each star has at least one planet, on average.

The Habitable Worlds Observatory with Evgenya Shkolnik - StarTalk Radio

(from 4:50)

Professor Shkolnik explains how we know this briefly, but what data exactly leads to this conclusion?

The First Exoplanets We Detected

Before getting into how we came to the estimate, it’s worth looking at the early hunt for exoplanets. The first confirmed discovery came in a 1992 paper, which looked at data from a pulsar, a rapidly-rotating type of star which produces a reliable, periodic signal pulse. This particular pulsar – the not-so-memorably named PSR1257+12 – should release a pulse every 6.2 milliseconds. However, precise measurement showed an unexpected, periodic variation in timing.

Researchers realised that this was likely due to a planet. As the pulsar is tugged slightly due to planetary orbits, the pulses’ travel time to Earth varied. In fact, they determined two exoplanets were probably responsible.

A similar but more broadly-applicable method is using the same effect of planetary orbits on a more regular star. The change in star motion relative to Earth, as it’s gravitationally pulled by an exoplanet, produces a Doppler shift in the wavelength – like the apparent change in pitch as an ambulance speeds past you. The first exoplanet discovery in a main-sequence star came in 1995 using this method.

Why It’s One Planet Per Star on Average

But these are just two methods – there are many more. One of the most interesting is gravitational microlensing – the same method that will be used by the Nancy Grace Roman space telescope – which depends on the magnifying effect of small-scale gravitational lensing.

The likely source of the “one planet per star” claim is a 2012 paper which used microlensing data from two surveys, the Optical Gravitational Lensing Experiment (OGLE) and the Probing Lensing Anomalies Network (PLANET), from 2002 to 2007. As Professor Shkolnik explained, the researchers combined detections – matched across both datasets – with calculations of detection efficiency to make the estimate.

They found that in the range of between 0.5 to 10 Earth orbital distances and between 5 Earth masses and 10 Jupiter masses, every star in the Milky Way has 1.6 planets on average. They write “Planets around stars in our Galaxy thus seem to be the rule rather than the exception.”