Free Astronomy Magazine September-October 2021

SEPTEMBER-OCTOBER 2021 51 Pegasi b, also called “Dimidium”, was the first exoplanet discovered or- biting a star similar to the Sun. This breakthrough discovery in 1995 con- firmed that planets like Earth might exist elsewhere in the universe, even though 51 Pegasi b itself is actually a “hot Jupiter,” something extremely dif- ferent from Earth. Despite this, its discovery earned its discoverers a belated Nobel Prize (in 2019). [NASA/JPL-Caltech] T he planetary systems of the Sun and 51 Pegasi. In the solar system, gas-giant planets, such as Jupiter, orbit far from the Sun. In 1995, Mayor and Queloz reported the discovery of 51 Pegasi b, a gas-giant planet that is much closer to its host star than Mercury is to the Sun. The orbital distances of the planets are given in astronomical units. The sizes of all objects are shown approximately to scale. [Nature] has reached levels of precision com- parable to those of the best existing spectrographs. As pointed out by the NEID team: “Our measurements of stable stars consistently show variability less than 1 meter per second. This on-sky stability reflects a combination of noise sources, including the instru- ment, statistical fluctuations (so- called ‘photon noise’), and the star’s inherent atmospheric variability. Thus, while it is hard to pin down an exact number, we are assured that NEID’s instrument-limited measure- ment precision is significantly better than 1 meter per second.” Jason Wright comments: “In the last decade, the state of the art has been roughly 1 m/s. NEID is ex- pected to reach 0.3 m/s, pushing the envelope to higher precision. When we combine future NEID observa- tions with data from spacecraft, things will really get interesting, and we will be able to learn what planets are made of. We will know the planet’s density, which is a clue to understanding how much of an atmosphere the planet has; is it gaseous like Saturn, an ice giant like Neptune, rocky like Earth, or some- thing in between — a super-Earth or sub-Neptune?” However, reaching an accuracy of about 0.3 m/s in the measurement of radial velocities does not mean easily discovering other Earths around stars as large as the Sun, but it will certainly be possible to dis- cover them around slightly smaller stars. Let us consider that Jupiter at- tracts the Sun to itself at a speed of 13 m/s, a value that for Earth drops to just over 0.08 m/s − just 8 cm/s − not exactly close to the theoretical