Searching for weather patterns on free-floating worlds

by Johanna Vos, University of Edinburgh

Planet 1

Artist’s conception of a free-floating planet. Image: NASA

At the start of 1995, we knew of only 9 planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Although we have since lost Pluto, we have now confirmed over 3,700 exoplanets — planets orbiting a star other than our sun. These exoplanets have been discovered by various methods, but the vast majority have been detected via indirect methods — measuring the influence of an exoplanet on its host star. We have also managed to directly image a number of exoplanets. This is the most difficult technique since most planets are lost in the bright glare of their host star. In recent years, we have discovered that in addition to the exoplanet companions — exoplanets orbiting a host star, there have been a number of discoveries of so-called rogue, or free-floating planets. These are planetary-mass objects (less than about 13 times the mass of Jupiter) with no host star, wandering the Milky Way alone!

There are currently two theories about the formation of these isolated planets. The first theory suggests that they form similar to a star like our Sun — through the collapse of a massive interstellar cloud composed of molecular gas and dust. Once enough material is compressed at the centre of the cloud, nuclear fusion is ignited in the core, and a star is born. Once nuclear fusion is established, a star will continue to shine for about 10 billion years. However, in the case of our free-floating planets, we think that the core did not accrete enough material to trigger nuclear fusion. These objects can be thought of as ‘failed stars’, and spend their entire lifetimes cooling down. The other theory proposes that the free-floating planets were ejected from a planetary system. This can happen due to gravitational interactions with other planets within the system or a close encounter with another star. These interactions could fling a planet out of its orbit and leave it free to travel through interstellar space. Most likely, the free-floating planets that we have discovered to date formed through both of the theories discussed here, but we have not yet found a way to distinguish them from each other.

Free-floating planets pose a huge advantage to astronomers studying exoplanets. The population of free-floating planets share a remarkable resemblance with the small population of directly-imaged planets that we have discovered. The free-floating and companion exoplanets share similar masses, temperatures, ages and sizes, but while the companion exoplanets are extremely hard to image, the isolated planets are much easier since they do not have a bright host star nearby. New instruments and technologies are currently being developed so that we may study companion exoplanets in detail in the future. In the meantime, the free-floating objects can be studied in exquisite detail and act as useful analogues for the directly-imaged companions, providing clues on what we might expect.

Brightness Modulations Signal Atmospheric Features

While it can take several hours to obtain an image of an exoplanet orbiting its host star, a medium-sized telescope can capture images of a free-floating planet on ~5 minute timescales. We cannot resolve the surface of a planet since it is too far away, but we can make use of the fact that they rotate to try to identify the presence of weather patterns in the planet’s atmosphere. This is done through a technique called ‘photometric variability monitoring’, which basically means measuring the brightness of an object over time. By monitoring the brightness over many hours we can approximate what the upper atmosphere of such an object looks like. The video below shows an artist’s concept of a brown dwarf with atmospheric bands of clouds, thought to resemble the clouds seen on Neptune and the other outer planets in the solar system. The dots on the bottom show the measured brightness of the planet over time, called the lightcurve of the planet.


Artist’s conception of a rotating free-floating planet with bands of clouds resembling those seen on Neptune. The dots on the bottom show the measured brightness of the object over time.

PSO-318.5-22: A Cloudy Free-floating Planet

In 2015 I used the New Technology Telescope in La Silla, Chile to observe the free-floating planet PSO J318.5-22. PSO 318.5-22 is a free-floating planet situated 80 light-years from earth, with a temperature of 800°C and a mass 7 times that of Jupiter. This object is unusually red compared to other objects with similar temperatures, and this is thought to be due to the presence of very thick clouds in its atmosphere. Using the images, we could measure the brightness of this object in each frame, and found that the brightness of this isolated planet changed by up to 10% over the course of 5 hours. Follow-up observations showed that the lightcurve is periodic, repeating itself every ~8.6 hours, indicating that this was the rotational period of the planet. Every 8.6 hours an atmospheric feature, most likely silicate clouds and iron droplets, would rotate in and out of view. This was the first detection of weather on an planetary-mass object, and hinted that these atmospheric features may be common on extrasolar planets.

We then went on to observe PSO J318.5-22 simultaneously using the Hubble Space Telescope and the Spitzer Space Telescope, which allowed us to track the brightness of our target in a variety of different wavelengths with unprecedented accuracy. The new lightcurves revealed that although all lightcurves showed brightness modulations in agreement with a 8.6 hour rotational period, the lightcurves obtained from the Hubble and Spitzer telescopes appeared ‘out of phase’. This means that when the planet appeared at its brightest in the Hubble images, it appeared very faint with the Spitzer Space Telescope, and vice versa. The Hubble and Spitzer Telescopes differ in the wavelengths they use — Hubble observations are in the near-infrared while Spitzer probes longer wavelengths in the mid-infrared. Different wavelengths are sensitive to different heights in the atmosphere of the planet — the Hubble telescope sees deep into the planet’s atmosphere while the Spitzer wavelengths only see the highest altitudes. The observed shifts between lightcurves suggest that we are observing different layers of clouds located at different vertical positions in the atmosphere. These types of observations have thus allowed us to explore both the horizontal and vertical cloud structure of PSO 318.5-22, a rogue planet lying 80 light-years away.


Future Exoplanet Companion Studies with JWST

Now that we have developed the technique of photometric variability monitoring, we hope to extend these studies to the directly-imaged exoplanet companions once the James Webb Space Telescope (JWST) launches. Due to be launched in 2020, JWST will revolutionise all fields of astronomy by providing unparalleled sensitivity to astrophysical signals at a wide range of wavelengths. JWST will allow us to extend the variability monitoring discussed above to exoplanet companions, such as the HR8799bcde planets shown below. This system of four planets, called HR8799bcde is so far the only multi-planet system that has been imaged. By re-observing the HR8799bcde system over a number of years, astronomers could track their movement around their host star. The four planets shown here share very similar properties to free-floating planets such as PSO J318.5-22, and so we expect that they will show similar brightness changes over time. Current telescopes cannot obtain images of these planets at the sensitivity and cadence needed to measure photometric variability, but JWST will allow us to carry out these measurements for the first time. 


Video showing four exoplanets orbiting their host star. The host star HR8799 harbours four super-Jupiters with periods that range from decades to centuries. Astronomers re-observed this system over a number of years to map out the orbits of these four exoplanets. Video: Jason Wang and Christian Marois.


Don’t go changing – Syria and the international law on the use of force

by Katie Johnston, University of Oxford.



‘Don’t Bomb Syria’ rally outside the House of Commons, London, April 16th 2018. Photo by Steve Eason.

In which situations may a State lawfully use force against another State? Is it possible that the law may change over time, so that what would previously be considered an unlawful use of force will come to be seen as lawful?

These questions were at the centre of the debate about the legality of the airstrikes against the Assad government in Syria, carried out by the US, UK and France on 13 April 2018.[1] Yet they also go to the heart of the modern jus ad bellum, the rules of international law that govern the use of force by States in their international relations. My research looks at how the law in this area is structured and aims to identify the processes by which the jus ad bellum can change.

International law, as a decentralised system of States, has no central legislative body that enacts binding laws for the international community. States become subject to international legal obligations primarily by entering into treaties and through the emergence of rules of customary law as a result of the practice of States.

The adoption of the United Nations Charter in 1945 brought about a paradigm shift in how international law regulated war. From a sovereign right and a legitimate policy choice, albeit subject to increasing restriction over the first half of the twentieth century, the use of force by States became subject to a comprehensive prohibition, set out in Article 2(4) of the United Nations Charter.[2] There are only two narrow exceptions to this prohibition, when force may lawfully be used: collective measures authorised by the Security Council under Chapter VII of the Charter, and the right of States to use necessary and proportionate force unilaterally in self-defence, if an armed attack occurs.

In addition to this treaty law rule in Article 2(4) which binds all UN member States, the prohibition on force also developed into a parallel rule of customary international law, binding on all States.[3] It is also widely accepted that the prohibition on the use of force has the status of a “jus cogens” norm of international law, from which no derogation is permitted. Although the existence, definition and content of this category of international legal rules remains contested, typically jus cogens norms protect the fundamental values of the international community and include, for example the prohibitions on genocide and torture.

Thus, the prohibition on the use of force is expressed in three forms: the treaty provision in Article 2(4) of the Charter, a parallel rule of customary international law, and a jus cogens norm. To draw an imperfect analogy with domestic law, imagine the same prohibition of murder existing in a criminal statute, as a common law rule developed by the courts, and as a provision in the Constitution.

This account is necessarily a simplified one that skips over the many debates surrounding the definition of force, self-defence, and the functioning of the collective security system that have arisen since the Charter’s adoption, and some of which remain unresolved. However, fundamentally, this is the international legal framework that regulates the use of force by States today: a comprehensive prohibition on the use of force, with two narrow exceptions.

It is against this backdrop that the UK government’s claimed legal basis for the airstrikes on 13 April 2018 must be assessed. In a legal position published on 14 April, the UK government asserted that “the UK is permitted under international law, on an exceptional basis, to take measures in order to alleviate overwhelming humanitarian suffering.”[4] This is almost certainly not correct under international law as it stands – as noted above, there are only two exceptions to the prohibition on the use of force, neither of which applies in this case. The UK  position is inconsistent with the text of the Charter and very few States take the view that a legal right of humanitarian intervention currently exists, while many more have explicitly stated that they do not recognise the existence of any such doctrine.

Indeed, for the UK position to be correct, it would need to be shown that the legal framework set out above had evolved since 1945 so that a new legal basis for the unilateral use of force by states had come into existence. This could be, for example, through a change to the prohibition so that it no longer banned force used for humanitarian purposes, or creation of a third exception of humanitarian intervention. However, given the complex structure of the law in this area, any argument that the jus ad bellum has changed faces a number of obstacles.[5]

First, as described above, the prohibition on the use of force exists in multiple legal rules. Therefore, even if a State could show that a new customary international law rule permitting humanitarian intervention had come into existence, any State claiming to rely on that right would still be in violation of its treaty obligation as a UN member not to breach the prohibition on the use of force in Article 2(4) of the Charter. This situation is further complicated by Article 103 of the UN Charter, which provides that the obligations of UN members under the Charter take precedence over their other obligations. Any State wishing to rely on a right of humanitarian intervention would therefore need to show not only that a new rule of customary law had emerged – which requires widespread and consistent practice by States accompanied by their belief that such a legal rule exists – but also that the UN Charter had been reinterpreted to allow for humanitarian intervention. Demonstrating such a reinterpretation, although possible in principle, is a difficult standard to meet: practice would need to establish the agreement of all UN members that the Charter should now be interpreted in this way.[6]

A second, more difficult, obstacle is presented by the jus cogens status of the prohibition on the use of force. Jus cogens norms can only be modified by another norm of the same character, so it seems that any new legal basis permitting humanitarian intervention would also need to show that it had fulfilled the test for emergence of a new jus cogens norm: acceptance and recognition by the “international community as a whole” that the norm has such a status.[7] This threshold is lower than that required to demonstrate reinterpretation of the UN Charter through subsequent practice, described above, but there are other characteristics of jus cogens norms that may complicate the process of change. Jus cogens norms invalidate contrary practice, depriving it of any legal effect it may have to bring about an evolution in customary law.[8] The presence of the jus cogens norm therefore seems to put those arguing for a change to the prohibition of the use of force in an all-or-nothing position: either they show that the stringent requirements for changing a jus cogens norm have been met, or any practice short of that threshold will not only be in violation of the jus cogens prohibition, but also ineffective to bring about a change in the customary or treaty norms.

In sum, the existence of multiple legal rules prohibiting the use of force, and the characteristics of the Charter and the jus cogens norm that cause them to prevail over conflicting rules, mean that changing the prohibition on the use of force appears to be very difficult indeed. The UK legal position does not explain whether the purported new right of humanitarian intervention on which it relies is understood as a new rule of custom, a reinterpretation of the Charter, or a new jus cogens norm. As shown above, it appears that a plausible claim may need to demonstrate that it is all three, simultaneously.


There are good reasons why it should be difficult to change the prohibition on the use of force. Conceptual and methodological arguments about legal rules gloss over the death, suffering and displacement that is caused by armed conflict; historically, the impact on women has been particularly overlooked.[9] The drafters of the Charter wanted to “save succeeding generations from the scourge of war”[10] by creating a new international legal order where unilateral use of force by individual States became the rare exception and not the rule. Even if the prohibition on the use of force is not universally complied with (what law is?), the current legal framework requires States to justify any use of force in terms of its requirements or face international condemnation, which in itself has a restraining effect. Creating a new legal basis for States to use force – without collective authorisation – would upset the current balance, tilting it in favour of greater unilateralism. In addition, in the specific context of humanitarian intervention, any increased permissiveness is unlikely to benefit all States equally. States that already enjoy greater military and political power will be more able to take advantage of a new right to use force unilaterally. Less powerful States, on the other hand, will see their protection from foreign intervention diminished.[11]

Nevertheless, the arguments remain finely balanced. While a legal right of humanitarian intervention does not exist in international law today, with the Security Council sliding back into gridlock and unable to address threats to peace and humanitarian disasters, international consensus may well shift to accept that intervention without Security Council authorisation is permissible in some circumstances. In such a case, it may be preferable for the international legal framework to be able to evolve to accommodate a new legal basis for the use of force, either within or in addition to the existing exceptions, rather than risk the prohibition being repeatedly violated or ignored, as appears to have been the case with the US, UK and French strikes last month. Developments such as the provision in the Constitutive Act of the African Union for a collective, regional, treaty-based right of humanitarian intervention could provide a less disruptive alternative to increased unilateral uses of force.[12] Yet, as the law stands, this provision appears to be unlawful.

Clarifying the processes by which the rules of the jus ad bellum can change could help provide a shared basis for States and international lawyers to evaluate claims that the rules governing the law on the use of force have changed, and avoid a more unstable situation where States instead abandon or undermine the authority of the prohibition on the use of force or the Charter system. It is this question, of how the prohibition on the use of force and its exceptions can change, that my research will  address.



[1] See, for example, Milena Sterio, Syria and the Limits of International Law, IntLawGrrls, 12 April 2018:

[2] Charter of the United Nations (1945):

[3] As later confirmed by the International Court of Justice, Militarv and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America). Merits, Judgment, I.C.J. Reports 1986, p. 14, at para. 190.

[4] Syria action – UK government legal position, 14 April 2018:

[5] As noted by Professor Dapo Akande in his legal opinion of 16 April: Akande, The Legality of the UK’s Air Strikes on the Assad Government in Syria, 16 April 2018:

[6] Vienna Convention on the Law of Treaties (1969), Article 31(3)(b):

[7] VCLT, Article 53.

[8] Orakhelashvili, Changing Jus Cogens Through State Practice? The Case of the Prohibition of the Use of Force and its Exceptions, in Weller (ed.), The Oxford Handbook of the Use of Force in International Law (OUP 2015).

[9] Hilary Charlesworth, Christine Chinkin, Shelley Wright, Feminist Approaches to International Law, 85 American Journal of International Law, 613, 645 (1991); Christine Chinkin, A Gendered Perspective to the International Use of Force, 12 Australian Yearbook of International Law 279, 293 (1988-1989).

[10] Charter of the United Nations, Preamble.

[11] See Anne-Charlotte Martineau, Concerning Violence: A Post-Colonial Reading of the Debate on the Use of Force, Leiden Journal of International Law (2016), 29, pp. 95–112.

[12] Constitutive Act of the African Union (2000), Article 4(h):