Ever since exoplanets were first discovered in the 1990s, astronomers have dreamed of finding an Earth-like planet amongst the stars. Better detection techniques have allowed us to find smaller and smaller exoplanets. But when we spot a planet beyond our Solar System, does “Earth-sized” really mean “Earth-like?” A new study presented at this week’s American Astronomical Society meeting shows that the smallest exoplanets are much more likely to be similar to Earth than we thought.

Astronomers, led by graduate student Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics, used the HARPS-North instrument to study Kepler-93b, an exoplanet 1.5 times the radius of the Earth.

HARPS HARPS stands for High-Accuracy Radial velocity Planet Searcher. The HARPS-N instrument is located on a 3.6-meter telescope in the Canary Islands and is used to confirm and follow up on exoplanet discoveries made by NASA’s Kepler Mission. The large number of exoplanets discovered in the past few years by Kepler were found using the transit method. The transit method looks for slight dips in a star’s brightness for evidence of a planet crossing in front of the star. Since the dip in brightness is related to the diameter of the planet, Kepler can get accurate diameters of exoplanets, but not their masses. HARPS stands for High-Accuracy Radial velocity Planet Searcher. The HARPS-N instrument is located on a 3.6-meter telescope in the Canary Islands and is used to confirm and follow up on exoplanet discoveries made by NASA’s Kepler Mission. The large number of exoplanets discovered in the past few years by Kepler were found using the transit method. The transit method looks for slight dips in a star’s brightness for evidence of a planet crossing in front of the star. Since the dip in brightness is related to the diameter of the planet, Kepler can get accurate diameters of exoplanets, but not their masses. HARPS-N, as stated in the name, uses the radial velocity method for studying exoplanets. As an exoplanet orbits around its host star, its gravitational tug on the star produces a slight “wobble.” That wobble can be detected by the Doppler shift of the star’s spectral lines as it moves ever so slightly towards and away from us. This method was famously used to discover the “hot Jupiters” early in the days of exoplanet measurements, as these massive planets very close to their stars produce a large Doppler shift. With refined techniques and instruments, such as HARPS-N and its predecessor HARPS in Chile, smaller planets with distances further from their stars can be measured. The radial velocity method, since it depends on gravitational tug, determines the masses of these exoplanets.

The latest study shows that Kepler-93b, an exoplanet with a diameter of 1.5 times that of Earth, has a mass of 4.02 times that of Earth. The mass and volume give a density, from which we can infer that this little exoplanet has a composition very similar to Earth’s.

In fact, by looking at all of the exoplanets with a diameter less than 2.7 times that of Earth’s and known masses, a pattern became clear. The smallest of these, 1.6 Earth-diameters or less, show a relationship between mass and size that reflect compositions similar to Earth and Venus, both of which have rocky mantles and metallic cores. The larger exoplanets, however, were less dense, and they have more materials like water, hydrogen, and helium. These planets don’t have analogs in our Solar System, but they're an interesting and diverse category in their own right.

Does a composition similar to that of Earth mean that the planet is truly Earth-like? There are other factors to consider before this determination can be made. Location in its system is another major factor that contributes to the environment on an exoplanet. For example, Kepler-93b, though Earth-like in composition, is in a very tight, 4.7-day orbit around its star. In order to support liquid water on its surface, an exoplanet needs to be at the right distance, and this one is much too close.

There are other, more nuanced factors in a planet’s history that will ultimately determine if it has liquid water or other conditions needed for it to be considered habitable by life as we know it. Nevertheless, having a composition similar to that of Earth is a good start.

Kepler-93b and its cohorts may not be the Earth-like worlds we are looking for, but their presence is a tantalizing hint that such worlds do exist in our galaxy. Kepler looked at just a tiny area of the sky, after all, and could only detect those planets with the right orbital geometry with respect to the Earth so that they can be seen in transit. Kepler’s planets are therefore just a small, hopefully representative, sample of what else is out there.

We’ve run into exoplanets and systems that were completely unexpected, but we’ve also learned, as we have time and time again in the history of science, that the Universe is much the same everywhere we look. Our “other Earth” may still be out there waiting to be discovered.