Cool Stars, Hot Exoplanets with Dr. Emily Rice

Clad in a striking purple Jupiter-atmosphere-themed dress, on March 23rd at Columbia University’s Pupin Hall, Dr. Emily Rice gave an at once informative, far-reaching and entertaining talk on exoplanets and brown dwarfs. As she pointed out in the course of her lecture, the purple Jupiter, rather than just artistic liberty taken by a fashion designer, is actually meaningful as a fair representation of the appearance of at least some brown dwarfs—in spite of their name.

Dr. Rice, who is a professor at the College of Staten Island and researcher at the American Museum of Natural History, as well as a leader of the research group BDNYC (Brown Dwarfs in NYC), began by providing her audience with some context to her research, both in terms of physics, explaining the stellar classification system (OBAFGKM), and distribution of such types of stars in our galaxy. As she pointed out, although large blue stars outweigh smaller stars in our galaxy in terms of aggregate mass, the smaller stars, particularly red M-dwarfs, vastly outnumber the larger ones. To put it in perspective, large stars today range up to about 100 solar masses, and small stars down to about 1/10 the mass of the sun.

It is the smaller stars which are key to the hunt for exoplanets. There are three primary methods used today for detecting exoplanets, typically in combination: the radial velocity method, the transit method, and direct detection.

In the radial velocity method, we measure the small wobble of a star toward us and away from us caused by the gravitational effects of an orbiting planet, using the Doppler effect on the star’s spectrum produced by the wobble. In the transit method, we measure the dip in overall luminosity received from a star as an orbiting planet passes in front of it, thereby blocking out a small percentage (~1%) of its light. This method, made famous by the planet-detecting satellite Kepler, depends on our line of sight relative to the star and its planets. Both of these methods are therefore indirect means for locating planets. In both cases we are measuring the planet’s effects on its star’s light as seen from the Earth.   In contrast, in direct detection, we block out much of the light from the star itself using a device called a coronagraph, thereby allowing us to see the faint light of an orbiting planet. The difficulty in direct detection is that the planets are so much smaller and fainter than their host stars, comparable to trying to see a firefly next to a working lighthouse.

Because large stars are so much brighter and more massive than their smaller cousins, a planet orbiting a large star will generate a lesser effect on its host star’s light than it would if it were orbiting a smaller star, whether measured by either the transit or the radial velocity method. For this reason, small, habitable-zone planets are easier to detect around small stars. Or, as Dr. Rice puts it, most potentially habitable exoplanets have been found around cool stars.

Dr. Rice then moved on to the principal subject of her research: brown dwarfs. A brown dwarf is an object of about 20 to 70 Jupiter masses, which is insufficient to ignite hydrogen fusion (necessitating about 75 Jupiter masses). They are planet-like in size and appearance, and can be considered to be objects “in between” stars and planets. They are also sometimes called “failed stars”. They nevertheless emit radiation, through leftover formation heat, and at times through the fusion of deuterium, which has a much lower mass threshold to ignite (about 13 Jupiter masses). But as deuterium is much rarer than hydrogen in our universe, deuterium fusion is not a reliable stellar fuel.

Although brown dwarfs are roughly Jupiter-size in volume, they differ from planets in terms of formation. A brown dwarf forms through the same stellar disk-protostar process as stars, just with less mass, whereas planets form in the disk around a star. Because their light is faint, brown dwarfs weren’t discovered until recently. In fact after the Alpha Centauri system and Barnard’s Star (an M-dwarf), our next three closest stellar neighbors are brown dwarfs.

In appearance, brown dwarfs are similar to gas giant exoplanets, except that they are much easier to find and study (because of the firefly problem). Their atmospheres may be similar as well to those of the gas giants. We now have defined three additional stellar classes for brown dwarfs, to add to the stellar letter classification system mentioned above: L, T, and Y dwarfs. Y dwarfs in particular, the coolest of the three (down to 25°C, cooler than the human body), emitting their radiation in the infrared portion of the spectrum, are typically depicted as purple in visible light color, hence the purple Jupiter dress!