Future of Astronomy from Recent History Extra Solar Planets

By its very nature – and perhaps more so than other sciences – the future of astronomy remains with what today remains unseen.  As an endeavor astronomy includes observation with instruments such as optical and radio telescopes plus precise measurements with sensors and cameras at sites in space and on the ground.   Beside observers and their instruments there are theoreticians striving to make sense of observations through models more and more computer based; and then there are engineers and manufacturers that provide astronomical equipment. There are also astronomy consumers who use its findings and there are astronomers involved in educating the public and astronomers of the next generation.   All of these factors should be considered in examining astronomy’s future which can be extrapolated from its recent past through research controversies, findings, capabilities and  current day limitations of its instruments. 

Textbooks directed at future astronomers and the general public provide clues about astronomy’s future.  Comparing old with new, we can sense directions or reflect on controversies that have led astronomy to where it now stands.  A hundred years ago astronomy books focused on celestial position measurements, constellations, brightness and magnitude of stars with only tentative notions of why the stars remained lit.  The size, motion and likely surface temperatures of planets and asteroids could be explained in terms of principles such as Kepler’s laws and radiative heat transfer, but little would be known about actual surface conditions or compositions and craters were an anomaly associated with the moon.  Over the decades more understanding was obtained of stellar energy sources and the physics of star outer layers.  Students learned to interpret the Hertzsprung-Russell diagram which described intrinsic stellar brightness and temperatures in terms of a star’s age, mass. and interior nuclear processes and that stars had varied lifetimes and varied fates.  In addition, after the 1920s, thanks to Edwin Hubble, it was discovered that stars in the hundreds of billions formed into galaxies; that these too had enormous numbers and that they were receding from each other faster and faster the farther out they were observed. 

Conclusions about cosmic origins and fate have changed considerably influenced by relativistic physics, behavior of subatomic particles and forms of matter and energy not easily detected, much less well understood.  Lunar craters no longer seemed exceptional but illustrative of how planets formed from the debris of circumstellar clouds of dust and gas.  Information on planets and the moon were supplemented by space probe images far exceeding earth-based telescope capabilities.  Then there were actual landings of astronauts on the moon and automated spacecraft on Mars, Venus and Titan…

With this quick survey some trends can be identified.  The advance of astronomy has depended very much on instrument capabilities.  Also, study of the solar system planets has rendered it not so much astronomy as geophysics, geography or atmospheric science that we once related solely to the Earth. Mars and Titan landing surveys are undertaken now by chemists and research biologists concerned with the precursors to life. Though such work is interdisciplinary these are not necessarily astronomers.  Astronomers will be more inclined to think of the moon as a platform for astronomy rather than an object of continued research.

As to the advance toward the unseen, in the 1970s when I was engaged in my own modest astronomical studies, there then existed the theoretical controversy about the existence of objects smaller than the smallest stars, objects 80 times more massive than Jupiter or starting at 8% the mass of the sun.  These stars were the smallest that had sufficient mass to set off the nuclear fusion processes of burning hydrogen into helium in their interiors that provided energy and illumination and these bodies were also considered the smallest that were likely to form from coalescing interstellar clouds of gas and dust.  Yet here we were living in a solar system with nine (or eight?) planets and orthodox astronomy concerned with stars was not sure it had an explanation for such circumstances as more than a fluke event near a star in a galaxy filled with 100 billion. 

After a symposium presented by one of my instructors, a specialist in stellar properties and especially in white dwarfs, (a diminished end state for many stars), in the break room we discussed the prospects of search for bodies which were less massive, objects which would have fallen between Jupiter and the smallest Main Sequence stars, the lower M types.  Based on his observing experience and theoretical knowledge, he was skeptical or pessimistic about the likely results.  These bodies would not be intrinsically bright even at birth and without nuclear ignition they would soon become even harder to detect as they continued to cool and shrink.

About the time we were discussing this prospect such bodies were tentatively theorized and subsequently given the name of “Brown Dwarfs” (for masses between 13 and 80 times Jupiter’s).  And later, as new methods of measuring doppler shifts in stellar spectral lines due to radial velocity changes were invented, smaller and smaller invisible partners to visible stars were detected by astronomers engaged in observing programs devoted to detecting possible planets.  Since 1995 hundreds of such brown dwarfs or even planets of less than the mass of Jupiter have been detected as orbiting partners of visible stars similar to the sun or less massive. Some have even been observed in visual or infra-red images; in some cases they have been observed in transit or have revealed details about their outer atmospheres.

And at this same moment in astronomical history when my instructor was saying that the as yet un-named brown dwarfs would be difficult to detect, astronomers were growing concerned over discrepancies between the mass in galaxies indicated by stellar luminosity and the mass indicated by the velocities of stellar orbital motion about galactic centers.  The latter suggested that an order of magnitude of more mass was lurking somewhere in the dark.  As it turned out, faint objects such as brown dwarfs and planets detected provided only a dribble of the mass needed to balance the scales.  More and more evidence was acquired that the matter missing from detection was of an exotic nature, perhaps not even made of atoms (non-baryonic) but something heretofore unobserved.

This leads up from the past to the present, but also indicates the path to the future.  These oversized planets are mostly very close to their parent stars; the doppler velocity shifts they impose are relatively large and cycle within a few days in most cases, though a few could be a year or more.  The selection of planets discovered tells as much about our instrumentation limits as the nature of extra-solar planets: these initial discoveries are the proverbial low hanging fruit.  To find a planet like the Earth will take  longer observing time and more sensitive instrumentation; some of that is already in operation on mountain observatories and on orbit with space telescopes like the recently launched  Kepler spacecraft.  

Subsequently, even when smaller and more earth-like planets are discovered, astronomers (not to mention the public) will want to know much more about them.  Do they have atmospheres similar to the Earth’s? Can we detect any signs of life or civilization?  Future research will attempt to identify faint spectral features in the light of intrinsically faint objects either by means already planned or yet to be devised. 

While as mentioned, some astronomers were skeptical about extra-solar planet detectability or existence, those that would have bet on them would have expected stellar planetary systems that were closer copies to our own.  Instead numerous bodies as large or larger than Jupiter seemed to have swum close toward their newly formed stars from outer reaches, using proto-planetary disk material surrounding them like a propulsive jet.   As dynamics of planetary formation were initially revealed in recent decades these strange detectable systems showed strange variations on solar systems; perhaps how they could form all wrong from our point of view or else how planets as big or bigger than Jupiter could go on a rampage.  The size, stability and chemistry of these disks from which planets are evidently formed are better understood now and research on these will continue along with efforts to detect planets themselves.

But as intrinsically interesting as the search for earth-like planets is, future astronomy has other lines of investigation.  Stellar behavior has been classified to a large degree and many types of stars are well understood, but it seems with each month there are new notices of strange stellar fates or behaviors.  Further characterization of dark matter and dark energy will certainly occupy astronomy’s future – as will detecting sources of neutrinos or gamma rays or gravity waves.  

Whether to categorize these other areas in terms of observational bands in the electromagnetic spectrum ( e.g., radio, millimeter, infrared, visible, ultraviolet, x-ray, higher energy…) or types of objects sought to observe ( normal or irregular stars, stars in birth, stars near death,  galaxies, galactic clusters, quasars, galactic centers…) , where astronomy might move in the future is away from areas where the reception of such signals and phenomena is reduced by the streetlights and radio noises created by our daily life.  This will be a retreat of a sort, true, but not necessarily a defeat; more like that of an ascetic going off into a desert to meditate on the cosmos.