Y Dwarfs: Exploring the Boundaries between Planets and Stars with JWST
What makes something a planet, a brown dwarf or a star? Generally, planets are less massive (<13 times as massive as Jupiter) and do not have enough central pressure to fuse deuterium, a form of hydrogen with an extra neutron. Stars are more massive (>80 times as massive Jupiter) and capable of fusing regular (neutron-free) hydrogen. The objects in between planets and stars are called brown dwarfs, which can burn only deuterium. However, the dividing lines between these objects are not always so clear – think about the controversy over how to classify Pluto, Kuiper Belt objects and dwarf planets in our own solar system. Another way to tackle this question is by how these objects form: are they made in a disk surrounding a star or do they collapse out of the gas in a star-forming cloud?
JWST will image and collect spectra of objects in all three regimes. Of interest will be low mass objects (5-10 times as massive as Jupiter) that float freely in space without orbiting a nearby star, and will be imaged with NIRCam’s filters as well as NIRSpec’s spectrograph. These objects are extremely faint due to their small sizes and low temperatures, with some only as warm as room temperature on their surface.
Objects with surface temperatures below around the boiling point of water are called Y dwarfs, which form the end of the stellar classification sequence OBAFGJKMLTY. Unlike hydrogen-burning stars at thousands of degrees, Y dwarfs have molecules in their atmospheres such as methane, carbon dioxide, water, and ammonia. JWST’s NIRCam imager will image these cool objects with its many filter bands to better understand their composition and temperature. Without Earth’s atmosphere, which is opaque at the many wavelengths of interest, JWST will be able to study the amount of molecules in Y dwarf atmospheres. JWST will also collect new information on the cloud heights and particle sizes in these atmospheres. The figure below shows the NIRCam and NIRSpec sensitivity as well as theoretical models of Y dwarf atmospheres.
