The star that nearly was

What gets smaller as it grows? Astronomer Dr Daniel Bayliss takes us through the latest Kepler telescope observations to get some answers… 

Not everyone can become a star. But EPIC201702477b came about as close as possible. This recently discovered object is known as a “brown dwarf”, and is a right on the cusp of turning into a star. If it had eaten up just a little more hydrogen when it was a young and growing quickly, it would have been able start nuclear fusion in its core – and it would shine as star for billions of years. However, it did not quite manage to get to this point – so it is fated to remain a dull brown dwarf for the remainder of its life. Nature can be so cruel...

In fact, EPIC201702477b is the densest object ever discovered – aside from stellar remnants such as white dwarfs, neutron stars, and black holes

Stuck as a perennial 'almost', just off being massive enough to be a star doesn't mean there is nothing special here though. In fact, EPIC201702477b is the densest object ever discovered – aside from stellar remnants such as white dwarfs, neutron stars, and black holes. Its density is a staggering 191g/cm3, over 10 times denser than gold. A teaspoon full of this material would weigh 1kg. EPIC201702477b was discovered thanks to NASA’s Kepler space telescope, a telescope specifically designed to detect exoplanets as they transit in front of their host stars. The telescope boasts a 1m diameter primary mirror coupled to a huge 95MP camera that covers 115 square degrees of sky. That is about 0.25% of the full sky.

Kepler was launched in March 2009, and worked on its primary mission staring at a single patch of sky for four years. However, in May 2013, the mission ended with the failure of a reaction wheel that meant Kepler could no longer point in a stable manner. But, were it not for this failure, the brown dwarf EPIC201702477b would never have been discovered. Because while this failure ended Kepler’s primary mission, some clever engineering allowed the telescope to continue work in a new survey named “K2”, which monitors fields around the ecliptic plane for about 80 days each. The spacecraft uses solar radiation pressure to stabilise its pointing, and although not as stable as it was during the primary mission, it is good enough to produce the highest precision light-curves currently attainable to astronomers.

These two “winks” were enough to alert scientists to the fact that something was transiting in front of this star – possibly an exoplanet

A quick dip One of the fields monitored as part of the K2 campaigns contained an unassuming star named EPIC201702477. The star is solar-like, about 1500 light years from Earth. It is not particularly bright, however while being monitored by the K2 mission its flux was seen to dip by less than 1% on two occasions. These two “winks” were enough to alert scientists to the fact that something was transiting in front of this star – possibly an exoplanet. Seeing an object transit twice in front of its host star is not common. Most of the transiting exoplanets discovered to date have been found by observing many ten’s or even hundred’s of individual transits. So to confirm this really was a transiting object, scientists used a network of ground-based 1m telescopes to catch the transit again. This helped confirm the transits were real and also allowed the timing of the transiting system to be refined. From these transits alone, it is possible to determine with some precision the size (i.e. radius) of the transiting object. However, the mass of the object could not be constrained without spectrographic observations.

Bigger yet smaller? A planet and a star orbit a common centre of mass, and therefore the stellar spectra can be monitored very precisely to observe the Doppler shift on the spectra. This provides a means by which the mass of the planet can be measured. By measuring the Doppler shifts of EPIC201702477 with large ground-based telescopes, it soon became apparent that the transiting object was too massive to be considered a planet. In fact the object is measured to be 69 times the mass of Jupiter. Conventionally, an object is only considered to be a planet if its mass is less than about 13 Jupiter masses. That limit is set as it is at that point that limited fusion of deuterium can begin. However this fusion is short lived and produces very little energy – thereby have little effect on the radius of the object.

So although EPIC201702477b is 69 times more massive than Jupiter, it is 20% smaller than Jupiter

Paradoxically, as a gas giant planet accretes mass it actually gets smaller, because the increased gravity of the object causes the material in the core of the plane to be degenerate. So although EPIC201702477b is 69 times more massive than Jupiter, it is 20% smaller than Jupiter. So what gets smaller as it grows? The answer is a brown dwarf! In a world where exoplanets are discovered on an almost daily rate (over 3000 are now know), these brown dwarfs are extremely rare. There are only 13 known transiting brown dwarfs – only 13 brown dwarfs that we can precisely measure their mass and radius. However these objects should be easier to detect than exoplanets, so why so few?

The scarcity of these objects is sometime known as “the brown dwarf desert”. It has been known for well over a decade now, although the reason for this desert is not clear. With this new discovery, some insight into the brown dwarf desert can be garnered. Firstly it has an orbit of 41 days, which while short compared with our own solar system, is actually significantly longer than most of the exoplanets we have found to date. And that is because it is much harder to find exoplanets in longer period orbits using traditional Doppler or transit techniques. So perhaps brown dwarfs just tend to reside further away from their host stars.

So it is possible that brown dwarfs sit in a mass-zone where neither planet formation nor star formation is very effective

One lump or two? Another idea as to why these objects are so rare relate to their formation. Most planets are thought to have formed by first growing a core, and then accreting gas on to that core. Such a process may be able to form planets such as Neptune or Jupiter, but it may not be able to produce objects as massive as EPIC201702477b. Conversely, stars form by the direct collapse of a giant gas cloud. This process can readily form stars like our sun, but is not able to easily form very low mass objects such as brown dwarfs. So it is possible that brown dwarfs sit in a mass-zone where neither planet formation nor star formation is very effective. This would account for the rarity of these objects. But can this theory be tested?

With only 13 known brown dwarfs with measured masses and radii, it is still a little difficult to study these objects as a population. However there are some hints from this small sample that there are actually two populations of brown dwarfs. The first are the low-mass brown dwarfs, with masses between about 13 and 40 Jupiter masses. These may have formed as planets but, for some reason, subsequently have grown much larger than usual. The second are around 60 Jupiter masses and great. These may be the very smallest objects from the star formation process. There is much work still to do to in order to figure out if we really have two populations of brown dwarfs, and the key will be to find more objects like EPIC201702477b.

Planetary treasure trove The prospects of more discoveries look promising. The K2 mission continues and will continue to monitor fields in the ecliptic plane over the next year or longer. Further into the future, NASA will launch the TESS satellite – the Transiting Exoplanet Survey Satellite. Although it is a much smaller space telescope than Kepler, it can monitor a huge section of the sky. In fact in just two years, TESS aims to monitor the entire sky, monitoring every bright star for at least 27 days. This will provide a treasure trove of new planets, but hopefully also a large number of transiting brown dwarf candidates. It will be an enormous amount of work to check whether the brown dwarf candidates that TESS finds are real and measure their masses via the Doppler spectroscopy technique. But when that is done, the idea of two populations of brown dwarfs should be able to be robustly tested, giving us clues into the processes of both planet and star formation.

Author: Dr Daniel Bayliss is an astronomer at Geneva Observatory and is part of the Swiss NCCR PlanetS organisation – dedicated to the study of the origin, evolution, and characterisation of planets.