NGTS discovers an extremely small star in an eclipsing binary

Guest blog by University of Leicester PhD student, and NGTS team member, Jack Acton

NGTS recently announced the discovery of an extremely small star, NGTS J0930-18B, in an unusual eclipsing binary system in work led by myself and the team at Leicester (Acton et al, 2020). Finding systems like this is quite rare, and although they might not command the same headlines as bona fide exoplanets, they are extremely important for the future of exoplanet science in general.

The system was discovered using the Next Generation Transit Survey (NGTS), a set of 12 telescopes operating at the ESO’s Paranal observatory in Chile, the home of the four 8m Very Large Telescopes (VLT).  The NGTS telescopes are much smaller however, just 20cm in diameter and survey patches of the sky for months at a time looking for extra-solar planets.  They do this by looking for the decrease in brightness when a planet passes in front of the star, blocking some of the light. The survey has been a great success, with 10 published planets and many more on the way.

Figure 1: The NGTS telescopes parked in their “Shed” at the Cerro Paranal Observatory in Chile. Image credit – Dan Bayliss

However, as well as detecting planets, NGTS is also fantastic at finding eclipsing binaries (systems with two stars in orbit around each other). When one star passes in front of the other it produces a drop in light similar to the effect of a transiting exoplanet. Indeed, determining which signals come from planets and which come from binary stars is a key part of exoplanet science. This is how we found NGTS J0930-18B.

When it was first observed by NGTS back in 2016, we noticed that the star, an otherwise ordinary M or red dwarf, periodically dimmed around every 1.3 days, strongly suggesting it was orbited by a companion of some kind – either a planet or another star. Using these data, combined with more eclipses observed by our team at the South African Astronomical Observatory, we determined that the object causing this drop in light would have a radius similar to that of Jupiter.

However, to figure out whether or not it was a planet we needed a key piece of information – its mass. We measure this by detecting the interaction between the star and its companion.  Just like how a star exerts a gravitational pull on the objects orbiting it, those objects exert a (much smaller) force on the star. By precisely measuring this we can determine the mass of the object orbiting the star. This required specialist observations with a bigger telescope, we used the High Accuracy Radial Velocity Planet Searcher (HARPS) instrument on the 3.6m telescope in La Silla, Chile. Using these measurements for NGTS J0930-18, we found that although the radius was similar to Jupiter, it has a mass that was just over 85 times larger! Meaning that rather than a planet, we had actually discovered an extremely small star.

Figure 2: Lightcurve of NGTS J0930-18. Note the clear drop in light from the star during the eclipse by the companion.

These tiny stars, known as late-type M-dwarfs, are extremely important, and the discovery of one as small as NGTS J0930-18B is fascinating. Stars of this type are known to be the most common in the galaxy, however they are rather poorly understood, particularly at the very lowest mass end. With a mass of 85 Jupiter masses, this is one of the smallest stars to have its’ mass and radius accurately measured (see figure 3) and is almost as small as it is possible for a star to be. Note that Jupiter itself has a radius about 1/10^th and a mass about 1/1000^th that of the sun. Below around 80 Jupiter masses, stars are no longer heavy enough to fuse Hydrogen, and instead become Brown Dwarfs, Jupiter sized objects with masses between 13 and 80 times that of Jupiter.

Figure 3: The mass and radius of NGTS J0930-18B compared with other, previously discovered and well characterised stars of similar size. NGTS J0930-18B is one of the smallest stars ever to have these parameters measured.

What’s even more unusual is that pairs of M-dwarfs in binary systems tend to have around the same mass. However here we have a normal M-dwarf (with a mass roughly 600 times Jupiter) and a companion that is extremely small. There are a few binary systems known containing pairs of M-dwarfs with such greatly differing masses, but none so extreme as NGTS J0930-18 (see Figure 4). It’s not entirely clear how this configuration has been able to form, posing questions for current binary star formation theory.

Figure 4: The mass ratio of known eclipsing M-dwarf binaries determined by dividing the mass of the smaller star by its larger companion, plotted as a function of orbital period. NGTS J0930-18 is shown in red and it is a clear outlier among the known population, with by far the most extreme difference in masses of any such binary yet discovered.

There are a growing number of intriguing planetary systems being discovered around low mass stars, the most famous of which is probably TRAPPIST-1, a system of seven Earth sized planets all of which have orbits shorter than that of Mercury. For us to properly understand these planets, and future planet discoveries, we must understand the stars they orbit. Eclipsing binary systems like NGTS J0930-18 are the best way of doing that because they allow us to precisely measure the component stars’ masses and radii independent of theoretical models.

Our paper ‘An Eclipsing M-dwarf close to the Hydrogen Burning Limit from NGTS’ has been accepted for publication In the Monthly Notices of the Royal Astronomical Society, and can be found on arXiv here – https://arxiv.org/abs/2008.07354