Science Nugget—Finding Habitable Earth-like Worlds Around Nearby Stars
  Steve Vogt, UCO/Lick Observatory

Over 200 planets have now been detected around nearby stars by using meter/second precision radial velocity observations from optical spectroscopy to sense the tugging of the planet on its host star. Any star hosting a planet wobbles in velocity, alternately being pulled first towards and then away from us by the orbiting planet, in strict Keplerian periodicity. The period and size of that wobble reveals the unseen planets' mass and as well as the size and shape of its orbit. The orbit size and brightness and temperature of the star are then easily combined to yield the equilibrium temperature of that planet.

If the planet sits just the right "Goldilocks" distance from the star (not to close, not too far) such that temperatures are in the 0-100C range, water can exist in liquid form on the planet's surface, and the planet is said to be in the star's "habitable zone". Most of the 200 known planets are not habitable, nor are they earth-like rocky balls, but rather are massive gas giants like Jupiter or Saturn. The holy grail of exoplanet hunters is a habitable earth-mass planet, one that is about the same size as Earth, and in the Goldilocks "habitable zone" around its star. Has nature made any "habitable earths" among the nearby solar neighborhood of stars?

Detection of a habitable Earth around a star like our own Sun is exceedingly difficult, as the wobble produced is at the level of only a few cm/second, far beyond current detection limits. However, if instrumented at first-light with a precision high resolution optical spectrometer, the TMT is easily capable of finding such "habitable Earths", even within the first observing season. The trick is to look at nearby M stars. These are much smaller in mass than our Sun, and show a much larger wobble due to any planets. And since these stars are cooler and less luminous, their habitable zones are much closer to the star, resulting in orbital periods of weeks to months instead of years, and offering the possibility of a quick detection.

Happily, nature apparently does make planets, even rocky Earth-sized ones, around M stars. Of the 200 presently known exoplanets a handful of rocky planets are already known around nearby M stars. Here is an artist’s rendering of the remarkable triple planet system around the nearby M4 dwarf star GJ 876.

The innermost (foreground) planet is only 5-6 times larger than the Earth, too close-in to be habitable, but other planets may yet emerge from this system, perhaps some even in the habitable zone. At a distance of only 15 light years, one could imagine having a two-way conversation with any such beings out in that system within a human life span!

Unfortunately, most M stars are just too faint to reach routinely with the kind of precision necessary, even with today's largest 10-m telescopes. However, the enormous light gathering power of the TMT will instantly bring into reach thousands of such M stars that can be quickly probed for habitable earths.

An instrument capable of providing the meter/second velocity precision necessary to detect such habitable Earth's is already on the drawing boards as a proposed early light instrument: the Moderate-to-High Resolution or MTHR spectrometer.

The following simulation illustrates how the MTHR + TMT combo could be used to detect a habitable Earth-sized planet. Here, a typical M2 star (with a hypothetical 5 Mearth[SD1] planet orbiting in a 50-day habitable zone orbit) is observed for 8 months, snatching a single quick 8-minute observation per night (probably in queue-scheduled mode) on 50 nights spread over a single season.

The resultant string of velocities in the top panel reveals (middle panel) a prominent spike of power at 49.54 days, nicely recovering the known input period and illustrating a very clear and convincing detection of this habitable earth-sized planet within the first commissioning season of TMT. Combining more observations over multiple seasons will further reduce the detectable mass limit, right on down to 1 Earth-mass.

There are over 10,000 M dwarfs within 200-300 light years, ready for reconnaissance for habitable Earths. Imagine the possibilities!