Exoplanets – 30 years of discovery
Penny Wozniakiewicz looks at advances in the field since the first worlds were announced
On 9 January 1992, astronomers Aleksander Wolszczan and Dale Frail introduced the world to the first two planets to be found outside the Solar System, alien worlds observed orbiting the pulsar PSR B1257+12, around 2,300 lightyears away. The hunt for ‘exoplanets’ – as they are also known – was then, as now, heavily driven by our quest to find out just how unique (or not) our Solar System is, and whether there is life beyond it.
Given the latter aim, finding exoplanets orbiting a pulsar – effectively a dead (no longer burning) star – is far from ideal. Rather fittingly, a competition run by the International Astronomical Union (IAU) resulted in the pulsar and its planets being named after various macabre characters from mythology and popular culture: Lich for the star, and Draugr, Poltergeist and Phobetor for its worlds (the third was discovered two years later). Nevertheless, the discovery spurred interest in strange alien worlds among the public and the scientific community, and now, after three decades of searching, some 4,500 confirmed exoplanets have been identified.
So many planets to choose from
And strange worlds they are indeed – the sheer variety observed among these exoplanets is vast. To date, they range from rocky terrestrial planets similar in size to Earth, to gas giants much larger than Jupiter; from extremely hot to exceedingly cold, with orbits taking them insanely close or excessively far from their stars. They have also been found around all sorts of stars – small, large, young, old and dead. Some even orbit multiple star systems, while so-called ‘rogue planets’ do not even orbit a star.
Both ground and space-based telescopes have been instrumental in exoplanet discoveries, perhaps most notably the Kepler Space Telescope, which is credited with more than 2,600 confirmed finds. Although it is possible to directly image some exoplanets (particularly those that are bright, massive and orbit at large distances from their star), the vast majority of exoplanets have been identified through indirect methods – the planets are found by studying the effect they have on their stars.
For example, the ‘radial velocity’ and ‘astrometric methods’ both search for evidence of wobbles in the motions of stars caused by orbiting planets, while the ‘transit method’ studies the dip in the brightness of stars caused as planets pass in front of them.
Combined, these studies allow us to work out details of each planet’s orbit and size, and then
determine whether it is a rocky terrestrial planet or a gas or ice giant, and whether it orbits within the star’s habitable zone. This is the region around a star where temperatures may permit the existence of liquid water on a planet’s surface – and from what we know about life, liquid water is an essential ingredient.
But finding a planet like Earth in the habitable zone of its star does not mean liquid water is present or that it will be habitable – key to the ability to host liquid water is the presence of an atmosphere to maintain sufficient surface pressure. Atmospheres also provide a means to explore the conditions present on these planets, as their compositions may reveal details of surface processes and even potentially the presence of life.
On the trail of exoplanets
Studies of exoplanet atmospheres are performed using spectroscopy. For a transiting exoplanet this involves measuring the intensity of the star’s light at different wavelengths as the planet passes in front. Gaps in the spectrum result from absorption by elements or molecules present in its atmosphere. ▲ Above left: a common way to detect exoplanets is by using the ‘transit method’; where a dip in the brightness of a star can indicate a passing planet
Above right: as it unravels the mysteries of early planetary systems, the James Webb Space Telescope (JWST) will be on the look out for new worlds
Several different atmospheric constituents have been identified in exoplanet atmospheres, including water vapour, carbon dioxide and methane, and scientists have even interpreted details such as the presence of clouds, rain and extremely high-speed winds on some.
Current studies estimate that trillions of planets could exist in our Galaxy, so it’s safe to say that we have barely touched the surface when it comes to finding and learning about exoplanets. As new telescopes come online from now and in forthcoming years, such as the long-awaited James Webb Space Telescope (JWST), the Nancy Grace Roman Space Telescope (2027) and the Atmospheric Remotesensing Infrared Exoplanet Large-survey (2028), they will surely identify more exoplanets and investigate their atmospheres – perhaps one day finding that exoplanet which is truly similar to Earth.
Dr Penny Wozniakiewicz is a planetary scientist and space dust expert based at the University of Kent
Take any photo of the night sky with a DSLR camera and after a certain length of exposure, the stars will start to trail due to the apparent movement of the stars (in reality, Earth’s rotation).
Not so long ago, you needed a large equatorial mount and some familiarity with the ‘dark art’ of image processing to track the movement of the stars. Today, though, there are many small, lightweight, star-tracking mounts that allow us to attach a DSLR camera with a lens or even a small telescope, and to follow the stars’ apparent movement to get extended exposures.
These portable mounts still have one important thing in common with their heavier counterparts: they require polar alignment to track the night sky successfully. Most star tracker mounts use a small polarscope to align on Earth’s axis of rotation, which, if you extend this line out into space, is slightly offset from the easy-to-locate Polaris (Alpha
(α) Ursae Minoris) – the ‘North Star’ or ‘Pole Star’. Aligning usually means placing the star roughly in the polarscope’s field of view and rotating the right ascension (RA) axis to the correct point, to position Polaris into a small circle on the graticule – the ‘clock face’ pattern that is seen through the eyepiece of the polarscope.
In some cases, you can align an engraved set of constellations on the graticule (such as Cassiopeia and Ursa Major) with their positions in the sky. Your level of success, however, is determined by how accurately you can perform the alignment for your equipment and requirements. A large system with a telescope needs to be more accurately aligned to track the stars successfully.
Longer exposures, deeper targets
Let’s look at how a DSLR star tracker can help increase exposure times with polar alignment. If you are using a star tracker and a camera with wide-field lenses (up to 35mm) then an approximate alignment with Polaris in the centre will typically produce exposures of a couple of minutes. In contrast, if you perform a proper polar alignment, you can push the exposures up to 10 or even 20 minutes with the same lenses, depending on the level of light pollution.
So, the ability to polar align is vital for getting the most out of your DSLR star tracker. In addition, the more accurately you can perform the polar alignment, the longer lenses and exposures you can employ, despite the increased weight on the system.
Our preferred method is to use either the SkyWatcher SyncScan Pro app (for Android and iOS, bit.ly/3yuVHCQ) or the iOptron Polar Scope app (for iOS only, apple.co/33gFBku) to show where to place Polaris on the graticule, based on the hands of a clock. By ensuring that the ‘0’ is lined up with the vertical axis of your mount while looking through your polarscope, you can then use either one of the apps to help you position Polaris correctly; just follow the step-by-step guide opposite.
Once you’ve followed the steps you can look forward to capturing great data and producing stunning images. And, as a bonus, you can also use the same method to align your heavier equatorial mount.