Five centimeters per second. As of today, that’s a holy grail. And—News Flash—it’s been achieved this February.
Headlines and Biblical references aside, what are we talking about? Those of us fortunate enough to attend a lecture by Dr. John Brewer at Columbia University’s Pupin Hall on February 15 found out.
The New York native is an Associate Researcher at Yale University’s astronomy department, where he specializes in exoplanets, solar and stellar astrophysics, and asteroseismology. He is also a founding member of Planet Hunters (http://www.planethunters.org) and a member of Yale’s 100 Earths Project, which was the subject of his talk.
The 100 Earths Project http://exoplanets.astro.yale.edu/science/100_Earths.php) is, simply put, a search for 100 Earth-like planets in the Sun’s neighborhood. Despite its straightforward description, this quest is not an easy one, and presents some serious engineering problems to today’s technology.
Brewer started by walking his audience through the terms of the famous Drake Equation. Since Frank Drake postulated the equation in 1961, we’ve discovered thousands of planets, primarily through two methods. The more recent of the two is the transit method, made famous by the Kepler satellite and continuing to be used by its successor satellite, the TESS mission, whereby a satellite stares at a portion of the sky and measures the tiny dips in brightness from each star when one of its planets transits between the star and us. Kepler’s objective was to measure the prevalence of planets in the Milky Way, and its success has now demonstrated that our galaxy is by any measure crawling with them, on the order of one planet per star on average. This translates to hundreds of billions of planets in our home galaxy.
The radial velocity method measures the “wobble” of stars, using the Doppler shift caused by a star’s slight movement toward and away from us as a result of being tugged upon by its attendant planets. Generally speaking, each atom has a cloud of electrons at various energy levels surrounding its nucleus; when an electron changes energy levels, it emits a photon at a very specific frequency corresponding to that specific energy level of a specific element. A star’s spectrum is therefore equivalent to a fingerprint, the spectral lines identifying which elements the star contains. So, as a star moves toward or away from us, the Doppler effect causes these spectral lines to shift slightly from their normal positions toward the red or blue ends of the spectrum, an effect which we can measure. As this measurement can be performed from Earth-based telescopes, this method predates Kepler. Applying advanced mathematics to the measured shift yields us not only the planet’s orbital period around its star, but also its mass, something that the transit method cannot do. Knowing the mass of a planet tells us something about whether it is likely to be rocky or gaseous.
The first exoplanets discovered via radial velocity were what we now call “hot Jupiters”—gas giants orbiting close-in to their stars. This was a question of selection bias: a massive planet tugs more strongly on its host star than does a small rocky one; and the closer the planet is to its star, the greater its tug. So hot Jupiters were the easiest planets to detect, producing a large stellar wobble of 60 meters/second in the case of 51 Pegasi b, the first exoplanet discovered (in 1995) orbiting a main sequence star and certainly not a place you’d like to visit!
By comparison, the wobble in our Sun’s motion produced by Jupiter’s gravity is about 12 m/s, while the effect of Earth’s gravity, given its closer proximity but far smaller mass, is only about 0.085 m/s (or 8.5 centimeters/second).
This means that in order to detect Earth-like exoplanets, we need to achieve a precision in measurement of stellar wobbles smaller than 10 cm/second, which has to date never been accomplished.
Aha! Yes, that’s what the five cm/second reference was all about. Using a new telescope at Lowell Observatory in Arizona (the Discovery Channel Telescope) and a new extremely precise spectrometer, the 100 Earths team has, as of the start of February, achieved a precision of 5 cm per second, which is theoretically enough to find Earth-like planets around nearby stars.
Dr. Brewer had just returned from Lowell Observatory with the late-breaking news. He then walked us through the engineering issues involved in achieving such a goal. First, the extreme precision spectrometer (“Expres”) requires isolation from temperature changes, air pressure changes, and vibrations of any sort from its ambient surroundings—not at all an easy task for an instrument of this size and weight. Furthermore, Expres needs extreme precision in measuring pixel locations to pinpoint the absorption lines in each spectrum. These are the challenges which the 100 Earths team has overcome, reaching an “instrumental” precision of 5 cm/second.
In addition to those engineering challenges, there are also astrophysical challenges, which include compensating for the Earth’s movement around the Sun, removing the artifacts of water and oxygen lines in the Earth’s atmosphere, compensating for the changing thickness of the Earth’s atmosphere, and the boiling motion in the atmosphere of the star being studied. These factors lower the precision of radial velocity measurements, despite the technical achievements at the engineering level, and the 100 Earths team is currently working on reducing these to a minimum.
So where do things stand? The 100 Earths team hopes to achieve a precision of 10 cm/second in practice on stars—half the instrumental precision of Expres—which they hope will be sufficient to identify terrestrial planets in the habitable zone of nearby stars. Going forward, we should expect great things from the 100 Earths Project!